There are so many myths and misconceptions surrounding fishkeeping.

Some have been passed down for generations, while others come from outdated information, pet shops, social media, or simply things that sound like they make sense.

The problem is that some of these myths can actually lead to poor aquarium conditions and unhealthy fish.

From believing that fish only grow as large as their tank, to thinking a fish can happily live in a bowl without filtration, there are plenty of common aquarium myths that are worth clearing up!

In this guide, we're going to look at some of the most common fishkeeping myths, explain what’s actually true, and help you make better decisions for your aquarium.

 

“Fish Only Grow as Big as Their Tank”

❌ Myth

One of the most common things you’ll hear in fishkeeping is that a fish will simply stop growing once it reaches the size of its aquarium.

Unfortunately, this isn't how fish growth works.

 

✅ The Truth

Fish growth is influenced by genetics, nutrition, water quality, temperature, health, and their environment, but a small aquarium does not simply tell a fish how large it is allowed to become.

When fish are kept in unsuitable environments, poor growth can occur, but this isn't healthy or desirable. Chronic poor conditions can cause stunted development, deformities, organ problems, and a reduced lifespan.

This is particularly important when buying fish that look small in the aquarium shop but can grow significantly larger as adults.

A fish that is only 5 cm when you buy it may eventually become 20, 30 or even 50 cm depending on the species!

 

• Tip: Always research the adult size of a fish before buying it, rather than choosing a tank based on the size of the fish at the time of purchase.

 

• Pro Tip: Don't ask “How big is this fish?”, ask “How big will this fish become?”

“Fish Don't Need a Heater”

❌ Myth

Some people believe that fish don't need heaters because their aquarium water is already at room temperature.

 

✅ The Truth

Many popular aquarium fish are tropical species and require consistently warm water to remain healthy.

Your home's room temperature can fluctuate considerably throughout the day and throughout the year. Even if the room feels comfortable to us, the temperature may be too cold for tropical fish.

Temperature affects fish metabolism, digestion, immune function and behaviour.

This is why maintaining a stable temperature is generally much more important than simply having water that feels warm enough.

However, not every fish needs a heater!

Cold-water species such as goldfish have very different temperature requirements to tropical fish.

 

• Tip: Always research the temperature requirements of your specific fish before deciding whether your aquarium needs a heater.

 

• Pro Tip: Keep a reliable aquarium thermometer in your tank even if you have a heater. A heater can fail, and your thermometer gives you an easy way to spot temperature changes before they become a problem.

"Betta Fish Can Live Happily in a Small Bowl"

❌ Myth

Betta fish are often sold in tiny containers, so it's easy to assume that they are perfectly happy living in a small bowl or vase.

 

✅ The Truth

While Bettas can survive in poor conditions for longer than many other fish, surviving is not the same as thriving.

Bettas still require clean, stable water, adequate swimming space, filtration and an appropriate temperature.

A larger aquarium also provides more stable water conditions and gives your Betta somewhere to explore and establish its territory.

Their labyrinth organ allows them to breathe atmospheric air, but this does not mean they don't need clean, oxygenated aquarium water.

 

• Tip: Give your Betta a properly sized aquarium with gentle filtration, suitable heating, hiding places and plants.
  

 

A fish being able to survive in a bowl doesn't mean a bowl is an appropriate home.

Think of it like being stuck in a tiny bathroom with an open toilet, with no way to leave, forever. You might survive, but that doesn't mean you'd consider it a suitable place to live.

 

“Fish Can Live in a Bowl Without a Filter”

❌ Myth

Because fish live in water, some people assume that simply putting water in a container and adding a fish is enough.

 

✅ The Truth

Fish constantly release waste into their environment through their waste  (peeing and pooing) and respiration. Uneaten food and decomposing plant matter also contribute to the aquarium's waste load.

This waste produces ammonia, which is highly toxic to fish.

This is where the nitrogen cycle becomes extremely important.

Beneficial bacteria colonise surfaces within the aquarium and help convert toxic ammonia into nitrite and then nitrate.

Filtration provides an excellent surface for these beneficial bacteria to colonise while also helping remove particles and circulate the water.

A filter isn't simply there to make the water look clean, it plays an important role in maintaining a healthy aquarium ecosystem.

 

• Pro Tip: If you'd like to understand exactly how ammonia, nitrite and nitrate work together, check out our Understanding the Nitrogen Cycle guide.

“Clear Water Means My Aquarium Is Healthy”

❌ Myth

If your aquarium looks crystal clear, it must be healthy, right?

Not necessarily!

 

✅ The Truth

Water can look perfectly clean while containing dangerous levels of ammonia or nitrite.

You cannot reliably determine your water chemistry simply by looking at it.

Likewise, cloudy water doesn't automatically mean that your fish are in immediate danger. Cloudiness can have several causes, including bacterial blooms, suspended particles or substrate disturbance.

This is why water testing is such an important part of aquarium keeping.

 

• Tip: Regularly test your aquarium water, particularly when setting up a new aquarium, adding fish, or whenever something seems unusual.

 

“Bristlenose Plecos and Other Catfish Will Clean My Tank for Me”

❌ Myth

Bristlenose Plecos and other catfish are often sold as “clean-up fish”, which can give beginners the impression that adding one to their aquarium will take care of algae, uneaten food and waste for them.
Corydoras are sometimes included in this group too, but they aren't really algae eaters. They spend much of their time searching through the substrate for food and will happily eat leftover food and small bits of debris, but they shouldn't be considered an algae-cleaning fish.

✅ The Truth

Bristlenose Plecos do eat algae and biofilm, but they are still fish, and fish produce waste.

In fact, Bristlenose Plecos can produce a surprisingly large amount of waste because they spend much of their time grazing and eating.

Other catfish have their own dietary needs too. Some species will graze on algae and biofilm, while others are more interested in sinking foods, uneaten food or other natural food sources.

 

Yes, some catfish eat algae, but the types and amount of algae naturally growing in your aquarium usually aren't enough to provide them with a complete diet.

 

Think of it like eating nothing but lettuce forever.

Yes, lettuce is food.

Yes, lettuce is good for you.

But is eating nothing but lettuce enough to provide everything your body needs?

 

No!

 

The same principle applies to your fish. Algae can be an important part of the diet for some species, but it shouldn't be assumed that the algae growing on your aquarium glass is enough to keep them healthy.

Bristlenose Plecos, for example, benefit from a varied diet that can include algae-based foods, sinking wafers or pellets, suitable vegetables and natural grazing opportunities such as driftwood.

They also require proper hiding places and good water quality.

Adding a Bristlenose or other catfish to a small aquarium simply because you have algae or excess food can actually create more maintenance rather than less.

It's also important to remember that uneaten food and fish waste don't simply disappear when another fish eats them.

The food has just moved through the aquarium's food chain and eventually becomes waste from the fish that consumed it.

 

• Tip: Treat every fish as another living animal that contributes to your aquarium's bioload, not as a cleaning tool. Choose fish because you genuinely want to keep and care for them, not because you want them to do your maintenance for you.

 

• Pro Tip: Before adding a “clean-up fish”, research exactly what that species eats and how large it becomes as an adult. You may find that the fish needs more food, space and filtration than you originally expected!

 

If you're unsure what to feed your fish, our guide How to Choose the Right Food for Your Fish covers the different types of aquarium foods and how to choose a suitable diet for your fish.

If you're specifically keeping Bristlenose Plecos, our How to Care for Bristlenose Plecos guide goes into much more detail about their diet and care.

If you're looking for suitable foods for your aquarium, you can browse our All Fish Food Collection.

“More Fish Means a Happier Aquarium”

❌ Myth

It can be tempting to think that adding more fish will make your aquarium feel more natural or give your fish more company.

 

✅ The Truth

It depends entirely on the species.

Some fish are naturally social and should be kept in groups, while others are territorial or solitary.

Adding too many fish can also overwhelm your filtration system and cause water quality problems.

Even if your fish aren't fighting, overcrowding can create chronic stress and poor water conditions.

 

• Tip: Research the social requirements of each species before deciding how many to keep.

 

Some fish need groups, while others may actually prefer their own space.

 

“You Need to Replace Your Filter Media Regularly”

❌ Myth

Many filter manufacturers sell replacement cartridges and recommend replacing them regularly, which can make it seem like all filter media needs to be thrown away and replaced.

✅ The Truth

Not all filter media is disposable.

Your biological media becomes home to beneficial bacteria, and replacing all of it at once can remove a significant portion of your aquarium's biological filtration.

Mechanical media such as filter floss (or wool) may need replacing when it becomes worn or clogged, but biological media such as ceramic media and sponge can often be cleaned and reused for a very long time.

Many built-in filters that come with aquarium kits, as well as some lower-quality filters, use what are essentially disposable filter pad cartridges. These can work, but they aren't always the most effective or economical way to filter an aquarium long-term.

The good news is that many of these filters can be improved by replacing the disposable cartridges with proper reusable filter media, such as sponge, ceramic biological media and filter wool, provided there is enough space inside the filter to do so.

This gives you much more control over your filtration and means you aren't constantly throwing away media that has become home to beneficial bacteria.

 

• Tip: If your filter only accepts disposable cartridges and you have no practical way to replace them, don't simply remove the old cartridge and immediately throw it away. A new cartridge starts with very little to no beneficial bacteria on it. Instead, seed the new cartridge first by placing it somewhere with good water flow in the aquarium or filter for a period of 3-4 weeks, allowing beneficial bacteria to begin colonising it before the old cartridge is removed.

 

• Pro Tip: If your filter is designed to hold two cartridges, replace only one at a time. Leave the other established cartridge running so it can continue providing biological filtration while the new media becomes established. This is particularly important in established aquariums, where suddenly removing all your established filter media can cause a major reduction in biological filtration.

 

Remember: Your filter isn't just a machine that catches dirty water, it's also home to a huge population of beneficial bacteria that help keep your aquarium's water safe and healthy.

 

“Algae Is Always Bad”

❌ Myth

The moment someone sees algae in their aquarium, their first thought is often:

“I've got an algae problem!”

But algae itself isn't necessarily bad.

 

✅ The Truth

A small amount of algae is completely normal in an aquarium and can even provide a natural food source for some fish and shrimp.

The problem is when algae grows excessively and begins covering plants, blocking light or becoming difficult to control.

Algae growth is influenced by several factors including light, nutrients, CO₂ availability, plant growth and aquarium maintenance.

Trying to eliminate every tiny piece of algae isn't always necessary.

 

• Tip: Instead of trying to completely eliminate algae, focus on maintaining a healthy balance between light, nutrients, CO₂ and plant growth.

 

If you're struggling with algae, check out our guide What Is This Green Stuff in My Tank? A Guide to Algae and How to Maintain It.

 

“If a Fish Is Sold at the Pet Shop, It Must Be Suitable for My Aquarium”

❌ Myth

If a fish is sitting in a tank at your local aquarium shop, it must be suitable for your aquarium at home.

 

Right?

 

✅ The Truth

Not necessarily!

Aquarium stores can stock hundreds of different species, each with completely different requirements.

A fish may require:

• A much larger aquarium
• Different water temperatures
• Different pH or hardness
• A specific diet
• A particular group size
• Special filtration
• Specific tank mates
• Different aquascaping requirements

Just because a fish is available for purchase doesn't mean it is suitable for your aquarium.

 

“But I Saw It Living in a Tiny Tank at the Shop?”

This is where things can get a little confusing.

You may walk into an aquarium shop and see large numbers of fish being kept in relatively small tanks. It can be easy to assume that this means those fish are happy living in that size aquarium.

 

But there's an important difference between a retail tank and a permanent home.

 

Fish in aquarium stores are generally being housed temporarily while they wait to be purchased and moved into their permanent homes. Some fish may sell within days, while others may take longer, but the intention is not for them to live in those display tanks permanently.

Think of a pet shop aquarium as a temporary waiting room, not the fish's forever home.

Of course, this doesn't mean fish should be neglected while they're in a store. Good filtration, water quality, stocking practices and regular maintenance are still extremely important.

What About Betta Fish?

Bettas are a great example of how this can be misunderstood.

You may see Betta fish being displayed in very small individual tanks and think:

 

“If the shop keeps them in that size tank, that's all they need.”

This isn't necessarily the case.

Some aquarium shops use systems where many of these smaller display tanks are connected to a much larger sump and filtration system. The individual tanks may provide limited swimming space, but the water is still being circulated through a much larger filtration system.

That doesn't suddenly make a tiny tank an ideal permanent home, but it does mean that what you see isn't always as simple as “this fish lives in this tiny tank”.

You may also find shops that keep Bettas in individual small containers with no shared filtration. Personally, this isn't a setup I'd recommend as a long-term environment for a Betta.

Unfortunately, the widespread use of tiny Betta containers may have helped reinforce the idea that Bettas require very little space.

 

• Tip: Don't use the size of a fish's retail tank as your guide for choosing its permanent aquarium. Instead, research the species' adult size, behaviour and environmental requirements.

 

Research Before You Buy!

It's much easier to decide whether a fish is suitable before bringing it home than trying to redesign your entire aquarium afterwards.

A fish that looks perfect in the shop might eventually outgrow your aquarium, require different water conditions, need to be kept with a group, become territorial, or simply have completely different care requirements from the fish you already keep.

 

• Pro Tip: Research the fish before you buy it, not after. Check its adult size, minimum tank requirements, temperature, water parameters, diet, temperament and social needs before handing over your money.

 

And remember, the fact that a fish is available for sale doesn't mean you have to buy it. Sometimes the best thing you can do for a fish is leave it at the shop until you have an aquarium that is properly suited to it.

“More Food Means Faster Growth”

❌ Myth

If you want your fish or fry to grow quickly, simply feed them as much as possible.

 

✅ The Truth

Nutrition is extremely important for healthy growth, especially when raising young fish, but more food isn't always better.

For most aquarium fish, feeding a small amount once or twice a day is a good starting point. The important thing is to provide enough high-quality nutrition without constantly overloading the digestive system or the aquarium with excess food.

Fish don't need to be eating constantly just because they're willing to eat.

In fact, allowing your fish to have an occasional fasting day can give their digestive system a break and help reduce problems associated with overfeeding, such as bloating, constipation and internal blockages.

 

• Tip: For most healthy adult fish, consider skipping food for one day each week. A short fasting period is generally well tolerated by healthy fish and can be a useful part of a balanced feeding routine.

 

Small Fish vs Large Fish

Not every fish should be fed on exactly the same schedule.

Smaller fish and micro-species generally have faster metabolisms and smaller stomachs, meaning they often benefit from smaller, more frequent meals.

Larger fish can often go longer between meals because they have larger digestive systems and generally process their food more slowly.

This is why a feeding schedule that works perfectly for a large cichlid may not be appropriate for a tiny tetra or nano fish.

 

What About Fry?

Fry and growing juveniles are a little different.

Young fish are growing rapidly and have much higher nutritional demands relative to their body size. Small, frequent meals can help provide the consistent nutrition they need for healthy growth and development.

Rather than giving fry one huge meal, it's often better to offer several small feeds throughout the day.

In our own fish room, we often feed growing fry 2–3 small meals throughout the day, depending on the species and their stage of development.

 

However, there is a catch.

 

More frequent feeding also means more waste.

Uneaten food can quickly decompose, while the increased amount of food being processed by the fish also increases the overall waste load in the aquarium.

 

• Pro Tip: If you're feeding fry multiple times a day, be prepared to increase your maintenance routine if necessary. Keep a close eye on ammonia, nitrite, nitrate and overall water quality.

 

How Much Should You Feed?

A good general guideline is to provide only what your fish can reasonably consume within around 1–4 minutes, removing any uneaten food where necessary.

However, this shouldn't be treated as a strict timer. Different species feed at different speeds, and some fish naturally eat more slowly than others.

The goal is to learn how much your particular fish actually need.

 

• Tip: Start with a small amount. You can always offer a little more if your fish finish everything quickly, but it's much harder to fix the water quality problems caused by repeatedly overfeeding.

 

And remember: a fish begging for food doesn't necessarily mean it needs more food!

Many fish will happily continue eating whenever food is offered, even when they've already had enough. Especially Gold fish!

For more information about feeding different types of fish, including flakes, pellets, frozen foods, vegetables and sinking foods, check out our How to Choose the Right Food for Your Fish guide.

 

“Fishkeeping Is Just About Feeding the Fish”

❌ Myth

Feed the fish, top up the water and enjoy the aquarium.

If only it were that easy!

 

✅ The Truth

Fishkeeping is about maintaining an environment where your fish can live long and healthy lives.

That means considering:

✅️ Water quality

✅️ Filtration

✅️ Temperature

✅️ Nutrition

✅️ Tank size

✅️ Stocking levels

✅️ Aquascaping

✅️ Behaviour

✅️ Maintenance

✅️ Compatibility

Feeding your fish is only one small part of keeping them healthy.

A beautiful aquarium isn't necessarily a healthy aquarium, and a healthy aquarium isn't necessarily one that requires expensive equipment.

It's about getting the fundamentals right.

 

“Goldfish Only Have a Three-Second Memory”

❌ Myth

Goldfish have terrible memories and forget everything after a few seconds.

You've probably heard this one before!

The idea that goldfish have a memory span of only three seconds is one of the most persistent myths in the aquarium hobby.

 

✅ The Truth

Goldfish are actually much smarter than they're given credit for!

Research and aquarium experiments have shown that goldfish can learn and remember things such as feeding routines, visual cues and associations. They've even been trained to navigate mazes and complete simple tasks.

They can learn to recognise patterns and associate certain cues with things that happen in their environment, including when and where they are fed.

This means your goldfish isn't simply forgetting everything that happened a few seconds ago. They're capable of learning from their environment and remembering what they've learned.

 

• Tip: Try interacting with your goldfish during feeding time. With patience, many goldfish can learn to associate you, a particular location or even a feeding cue with food

 

This is also one reason why providing goldfish with an appropriately sized aquarium and an interesting environment is so important. They're not just decorative fish swimming around aimlessly, they're active animals capable of learning and exploring.

 

• Pro Tip: Give your goldfish an environment with plenty of swimming space and things to explore. A larger, appropriately decorated aquarium provides far more stimulation than a small bowl or bare container.

 

And perhaps most importantly, don't underestimate your fish! 🐟

 

Why These Myths Matter

Most of these myths probably started somewhere for a reason.

Maybe someone kept a fish successfully in a small aquarium for years. Maybe someone never used a heater and their fish survived. Maybe someone had a Bristlenose that genuinely did keep their glass spotless.

The problem is that one person's experience doesn't necessarily represent what is best for the species as a whole.

Fish are remarkably adaptable animals, but adaptability shouldn't be confused with ideal care.

Our goal as fishkeepers should be more than simply keeping a fish alive.

It should be about giving them an environment where they can thrive.

 

Final Thoughts: Keep Questioning What You Hear

Fishkeeping has changed dramatically over the years, and we're constantly learning more about the animals we keep.

If you've heard something that sounds strange, don't be afraid to question it.

Research the species. Look at its natural habitat. Understand its behaviour. Learn about its water requirements and adult size.

Most importantly, remember that “I've always done it this way” isn't necessarily the same thing as “this is the best way to do it.”

The more we learn, the better we can care for our fish.

 

• Pro Tip: If you're thinking about adding a new fish to your aquarium, research its requirements before bringing it home. A few minutes of research can save you months of problems later!

 

Happy fishkeeping! 🐟🌿

 

Frequently Asked Questions About Fishkeeping Myths

Do fish really grow to the size of their aquarium?

No. Fish do not intentionally stop growing because their aquarium is small. While poor conditions and inadequate nutrition can stunt growth, this is not healthy or something that should be relied upon. Always choose an aquarium based on the fish's expected adult size and needs.

 

Do fish need a filter?

Most aquarium fish benefit greatly from proper filtration. Filters provide mechanical filtration to remove waste and, more importantly, provide surfaces for beneficial bacteria that help process toxic ammonia and nitrite. Some very low-stocked setups can be maintained without a conventional filter, but this requires careful management and isn't generally recommended for beginners.

 

Do Bettas really need a large aquarium?

Bettas don't need an enormous aquarium, but they still need adequate swimming space, stable water conditions, filtration and appropriate heating. Their ability to survive in small containers shouldn't be confused with those containers being an ideal permanent home.

 

Can goldfish really remember things?

Yes! The idea that goldfish only have a three-second memory is a myth. Goldfish are capable of learning and remembering associations, routines and environmental cues. They can even be trained to complete simple tasks and navigate mazes.

 

Do Bristlenose Plecos clean the aquarium?

Bristlenose Plecos can help graze on algae and biofilm, but they don't clean an aquarium for you. They are still fish that require proper food, produce waste and need suitable water quality, filtration, hiding places and tank space.

 

Can Corydoras eat algae?

Corydoras aren't primarily algae-eating fish. They spend much of their time searching through the substrate for food and may consume leftover food and small particles, but they shouldn't be added to an aquarium as an algae-control solution.

 

Should I replace my filter media regularly?

Not necessarily. Many biological filter media, including sponge and ceramic media, can be cleaned and reused for a very long time. Mechanical media such as filter wool may need replacing when it becomes worn or clogged. Avoid replacing all established biological media at once, as this can remove a large amount of your beneficial bacteria.

 

Is tap water safe for aquarium fish?

Untreated tap water can contain chlorine or chloramine, which can be harmful to fish and beneficial bacteria. Tap water should generally be treated with an appropriate water conditioner such as Aquasonic Power before being added to an aquarium.

 

Does feeding more food make fish grow faster?

Not necessarily. Growing fish need good nutrition, but excessive feeding can lead to poor water quality, digestive problems and unnecessary waste. Fry and juveniles can benefit from smaller, more frequent meals, while healthy adult fish generally don't need to be constantly fed.

 

Can I buy a fish just because my local aquarium shop has it?

No. A fish being available for sale doesn't mean it's suitable for your aquarium. Always research its adult size, temperature requirements, water parameters, diet, temperament, social needs and minimum aquarium requirements before purchasing.

 

Can fish live happily in a bowl?

A fish surviving in a bowl doesn't mean a bowl is an appropriate permanent home. Bowls typically provide very little swimming space, limited filtration and poor water stability. Different fish have different requirements, but most aquarium fish will benefit from a properly sized aquarium with appropriate filtration and stable water conditions.

 

Are all fishkeeping myths completely false?

Not always! Some myths contain a small amount of truth but have been exaggerated or misunderstood over time. That's why it's important to look at the actual needs of the species you're keeping rather than relying on general rules or something you've simply heard before.

When in doubt, research the specific species you're keeping. Fish can have dramatically different requirements, and good fishkeeping starts with understanding the animal rather than following a one-size-fits-all rule.