How to Add or Change Substrate in an Established Aquarium

Changing or adding substrate to an already established aquarium doesn't have to be a difficult task. With the right preparation and care, you can successfully upgrade or refresh your tank's substrate without disturbing your plants, fish, or water balance, whether you're adding a nutrient-rich substrate like aquasoil or simply refreshing what's already there.

 

Here's everything you need to know to make the process go smoothly:

 

What Is Substrate in an Established Aquarium?

Substrate is the material that covers the bottom of your aquarium. It can be inert (like sand or gravel) meaning it doesn't affect water chemistry or provide nutrients, or nutrient-rich (like aquasoil), which provides essential nutrients for healthy plant growth.

Keep in mind that not all substrates suit all setups. Sand, for example, is too dense for most plant roots and works best in non-planted tanks.

In an established tank, substrate serves several vital functions:

 

Supporting Plant Growth: A healthy substrate supports the development of plant roots, giving them a stable base to anchor and feed from.

 

Aids in Filtration: Beneficial bacteria colonise the substrate and help break down waste, playing a key role in biological filtration.

 

Affects Water Chemistry: Some substrates like aquasoil can lower pH and soften water, which can be beneficial for certain plants and fish.

 

Enhances Aesthetics: Changing or adding substrate can transform the look of your aquarium, whether you're aiming for a natural planted scape or something more ornamental.

 

Why Changing or Adding Substrate Can Be Important

Adding or changing substrate can improve plant health, create a more stable environment for fish, and give your aquarium a fresh look. Specifically, it can provide:

 

Healthy Plant Growth: A nutrient-rich substrate like aquasoil can greatly improve root development, especially for heavy root-feeding plants such as Amazon Swords.

 

Improved Filtration: A well-maintained substrate supports the growth of beneficial bacteria, helping with biological filtration and keeping your tank clean.

 

Stabilised Water Chemistry: Some substrates help stabilise pH and water hardness, providing optimal conditions for both fish, plants and invertebrates.

 

Preparing Your New Substrate

Before adding any substrate to your established tank, it needs to be properly prepared to avoid disrupting your aquarium's ecosystem. The method varies depending on the substrate type:

 

Gravel: Place your gravel into a clean bucket and fill with water. Agitate with your hands to dislodge dust and debris, a hose works well here, just don't use too much pressure or you'll blast the substrate out. Drain and repeat until the water runs clear. Pour slowly when draining to avoid losing substrate.

 

• Tip: When draining the bucket, pour slowly and carefully to avoid losing substrate, we also advise to do this on the lawn outside rather than in a sink so you dont clog your drains, you'd be surprised how much can slip out if you rush it.

 

Sand: Same process as gravel, rinse repeatedly until the water runs clear. This step is especially important for sand, as skipping it will cause significant cloudiness in your tank. Pool filter sand or play sand you can get from most hardware stores is a great affordable alternative to aquarium-specific sand, and works particularly well for bottom-dwelling fish like catfish.

 

• Pro Tip: Pool filter sand or play sand is a great affordable alternative to aquarium-specific sand. It's fine and smooth, making it ideal for bottom-dwelling fish like catfish, and it rinses just as well.

 

Aquasoil / Nutrient-Rich Substrates: This type requires more patience. Place the aquasoil in a bucket, rest a plate or saucer on top to protect it, then gently fill with water. Let it soak for 24 hours, the water will turn cloudy, which is normal. Then begin testing for ammonia daily using a test kit. Ammonia spikes are expected and completely normal at this stage. Use a water conditioner like Aquasonic Ammo Gone or Seachem Prime to help neutralise it, and perform regular water changes on the bucket until ammonia drops to near 0 ppm. This process can take 6–8 weeks, but it's worth it, doing this in a bucket means your tank's water quality stays stable when you eventually add the substrate.

 

• Tip: When filling the bucket with water for aquasoil, rest a plate or saucer on top of the substrate first. This stops the water from hitting it too hard and turning the soil muddy before it even makes it into your tank.

 

Preparing Your Aquarium

Once your substrate is prepped and ammonia levels are under control, it's time to get your tank ready.

 

Step 1: Remove Sensitive Plants and Decorations. Gently remove any delicate plants or decorations to avoid disturbing them during the substrate change.

 

Step 2: Vacuum the Existing Substrate. If you're adding a new layer rather than doing a full replacement, use a gravel vacuum to remove debris from the existing substrate. This clears waste and organic material before the new substrate goes in. If you're doing a complete change, you can siphon the old substrate out entirely.

 

• Pro Tip: If you're doing a full substrate replacement, a siphon hose makes removing the old substrate much easier than scooping by hand, especially for sand.

 

Step 3: Check Water Parameters. If you're using aquasoil or a substrate that affects pH, make sure your current water parameters, pH, hardness, and temperature, are within a safe range for your fish and plants before proceeding.

 

Adding the Substrate

Now that both your substrate and aquarium are ready, it's time to add it in.

 

Step 1: Add the Substrate Gently. Use a bowl or cup to scoop the substrate and lower it just above the existing substrate (or tank floor if old substrate has been removed) before releasing it. Spread it out as you go. Avoid dumping it directly into the water, this causes cloudiness and with aquasoil, can trigger ammonia spikes in the water column.

 

• Tip: Lower the bowl or cup as close to the existing substrate as possible before releasing it. The less distance the substrate falls through the water, the less cloudiness you'll get.

 

Step 2: Aim for the Right Depth. For nutrient-rich substrates like aquasoil, aim for a 5–7 cm layer to give plant roots enough depth to establish and thrive. For inert substrates, depth is more flexible but should be enough to anchor plants securely.

 

Step 3: Replant and Reposition. Gently place any plants back into the tank and ensure they're securely anchored. For heavy root-feeding plants, position them where they'll have direct access to the nutrient layer beneath.

 

• Pro Tip: For heavy root-feeding plants like Amazon Swords, try to position them directly over the deepest part of your substrate layer so their roots have the best access to nutrients.

 

Option: Completely Strip the Tank

If you want a completely fresh start, for example, if your existing substrate has depleted its nutrients or you're switching substrate types entirely, you can strip the tank down fully.

 

Step 1: Remove Fish and Plants. Carefully move all fish into a temporary container or tank filled with water from the aquarium to minimise stress. Do the same for any plants you want to keep.

 

Step 2: Remove the Old Substrate. Scoop or siphon out the old substrate completely, then vacuum the tank bottom to remove any remaining waste or debris.

 

Step 3: Clean the Tank. With the tank stripped, it's the perfect opportunity for a deep clean. Wipe down the glass and decorations with a sponge or scraper to remove algae and build-up. Avoid soap or any chemicals, these can harm your fish when they're reintroduced.

 

Step 4: Add the New Substrate. Follow the preparation and adding steps above to set up your fresh substrate layer, whether that's aquasoil, sand, gravel, or a combination.

 

Monitoring Water Quality After the Change

Once the new substrate is in place, keeping a close eye on water quality is essential, especially in the first few weeks.

 

Step 1: Test Your Parameters. Test regularly for ammonia, pH, and nitrate. If you pre-soaked your aquasoil using the bucket method, your tank should stay relatively stable and your filter to should help keep clear up any small amounts of water quality fluctuations, but it's still worth checking daily at first.

 

• Tip: This is exactly why the bucket pre-soak method for aquasoil is so worthwhile, by cycling the ammonia out beforehand, your tank stays stable and your fish aren't exposed to dangerous spikes.

 

Step 2: Perform Regular Water Changes. 20–30% water changes will help dilute any ammonia or pollutants present in the water column and keep conditions stable while the substrate settles.

 

• Pro Tip: Use a water conditioner like Seachem Prime or Aquasonic Ammo Gone during water changes in the first few weeks. It helps neutralise any residual ammonia and keeps conditions safe while the substrate fully settles.

 

Step 3: Watch Your Fish. Keep an eye out for signs of stress, lethargy, sitting on the bottom, gasping at the surface, or flashing and scratching. These are signals that water parameters may be off and that a water change or parameter adjustment is needed.

 

Final Touches and Long-Term Care

Once water quality is stable and the substrate has settled, you can enjoy the benefits of a refreshed tank.

 

Step 1: Reintroduce Your Fish. If you removed fish for the change, reintroduce them gradually, especially if water parameters have shifted. Don't rush this step.

 

• Tip: Before releasing your fish back into the tank, float their container in the aquarium for 15–20 minutes to match the water temperature. Even small temperature differences can cause unnecessary stress.

 

Step 2: Fertilise if Needed. If you're using an inert substrate like gravel or sand, add root tabs or liquid fertilisers to support plant growth. Inert substrates provide no nutrients on their own, so supplementing is essential for a planted tank.

 

Step 3: Ongoing Maintenance. Continue monitoring water parameters regularly. Nutrient-rich substrates like aquasoil will gradually deplete over time and may need replenishing, keep an eye on plant health as an early indicator, usually they do last a fair few years, typically 2-3, depending on what type/brand and how much you have in your aquarium.

 

• Pro Tip: Keep an eye on your plants as an early warning system, slow growth or yellowing leaves often signals that your substrate is starting to deplete its nutrients before your water tests show anything obvious.

 

Conclusion

Changing or adding substrate in an established aquarium is very achievable when done with care and the right preparation. Whether you're boosting plant health with aquasoil or simply refreshing the look of your tank, following these steps will ensure a smooth transition and a happy, healthy aquarium. Take your time, monitor your water, and your tank will thank you for it.

 

• Pro Tip: If you're new to substrate changes, start with aquasoil for a planted tank or sand for a non-planted setup like an African cichlid or rock formation tank. Keep in mind that sand is too dense for most plant roots, so it's not ideal if you're wanting to grow plants. Most importantly, always research the needs and habits of the fish you keep before changing your substrate, what works for you aesthetically needs to work for them too. As you gain experience, you can experiment with different substrates or combinations to find the perfect fit for your setup.

 

Frequently Asked Questions On Adding New Substrate

What is substrate and why does it matter in an established aquarium?

Substrate is the material covering the bottom of your aquarium. It can be inert (like sand or gravel) or nutrient-rich (like aquasoil). In an established tank, substrate supports plant root growth, houses beneficial bacteria for biological filtration, influences water chemistry like pH and hardness, and contributes to the overall look of your aquascape.

 

Do I need to remove my fish to change the substrate?

It depends on the method. If you're adding a new layer on top of existing substrate, you can often leave fish in, just work slowly and carefully. For a full substrate replacement, remove all fish into a temporary aquarium or container filled with tank water to minimise stress.

 

How do I prepare new substrate before adding it to my tank?

Rinse gravel and sand thoroughly in a bucket until the water runs clear, this prevents cloudiness. For aquasoil or other nutrient-rich substrates, soak in a separate bucket for 24 hours, then test for ammonia daily and perform regular water changes until levels drop to near 0 ppm. This can take 6–8 weeks but is essential, aquasoil naturally releases ammonia when first wet, which can be harmful to fish if added directly to your tank. Using a water conditioner like Seachem Prime can help neutralise ammonia during this phase.

 

How do I add substrate without clouding my tank?

Add it slowly, one scoop at a time, using a bowl or cup to gently place it just above the existing substrate and spread it out. Avoid dumping it directly into the water. For aquasoil, resting a plate on top before pouring water helps prevent it from turning muddy.

 

How deep should my substrate layer be?

For nutrient-rich substrates like aquasoil, aim for 5–7 cm to give plant roots enough depth to establish. For inert substrates like gravel or sand, depth is more flexible but should still be enough to anchor plants securely.

 

How do I monitor water quality after adding new substrate?

Test regularly for ammonia, pH, and nitrate, especially in the first few weeks. Perform 20–30% water changes regularly to dilute any pollutants. Watch your fish for signs of stress like lethargy, gasping at the surface, or flashing, as these can indicate water parameters need attention.

 

Do I need to fertilise if I'm using gravel or sand?

Yes, inert substrates do not provide nutrients on their own, so supplement with root tabs or liquid fertilisers to support plant growth. Heavy root-feeding plants like Amazon Swords especially benefit from root tabs placed directly beneath them.