How To Change Substrate In An Established Tank?

Changing the substrate in an established tank is not that difficult, but it takes time. You’ll need to remove everything from the tank, including the fish and ornaments. Here’s a guide on how to change substrate in an established tank, plus some tips on how to keep your fish safe during the process.

When To Change The Substrate In Your Fish Tank

Here are some reasons to consider replacing the substrate in your fish tank.

  • If you no longer like how it looks or you’d like to try a different style or colour of substrate.
  • If you suspect that the current substrate is not good for your fish. For example, if the small fish are unable to burrow in it or if it allows toxins to build up.
  • If you are planning to change the fish you keep in your tank. You may need to get a different type of substrate to suit the new fish (e.g. changing from gravel to smooth sand for barbells and other small fish).
  • If you are planning to introduce plants into your fish tank. You’ll need to replace the current sand or gravel substrate with soil substrate that supports plant growth.
  • If the current substrate is old and discoloured.

How To Change Substrate In A Fish Tank – Step by Step Guide

How To Change Substrate In A Fish Tank - Step by Step Guide

1. Do the prep work

Don’t wake up one day and just decide to change the substrate in your fish tank. There’s some prep work involved to make sure the process goes smoothly and the fish are not harmed.

  • Decide which new substrate you want and order it in advance.
  • Make sure the fish tank is in good condition. Test the water for nitrites, nitrates and ammonia. Both nitrites and ammonia should be at zero and nitrates levels should be under 20 ppm. If the numbers are off, figure out what’s wrong and solve the problem before changing the substrate. It could be a case of overstocking, overfeeding, poor filtration or something else.
  • Change or clean the filter at least two weeks before you replace the substrate. This will give bacteria time to take hold on the filter media, which will reduce spikes of ammonia and nitrites when you put in new substrate.
  • Make sure you have all the tools you need at hand such as a scoop or dustpan to take out the old substrate and a hose and bucket to clean the new substrate.
  • You also need to figure out where you’ll keep the fish as you change the substrate. If you don’t have a holding tank, consider getting one. You can also use a clean bucket that doesn’t have a trace of detergent or other chemicals.
  • A day before the substrate replacement, do not feed the fish. This will reduce the amount of waste they produce in the holding tank, which will help keep them healthy for the short period they’ll be out of the main tank.

2. Clean the new substrate

Make sure the new substrate is ready to go in once you remove the old one.

Clean the substrate to get rid of dust and other fine particles that can cloud the fish tank and clog the filters.

The easiest way to do this is in a bucket. Place a running hose in the bucket and tip the bucket forwards slightly so that the dirty water flows out.

Keep doing this until the water runs clear.

3. Empty the fish tank and transfer the fish into a holding tank

Turn off the filter and transfer about half the water from the fish tank into the holding or the bucket.

Using a net, transfer fish into the holding tank. Also transfer any rocks, plants and ornaments into the same tank to help maintain their bacterial colonies.

But if some of the ornaments are too big, you can just put them aside somewhere else.

4. Scoop out the old substrate

Remove the old gravel or sand using a scoop and bucket. Vacuum the bottom of the pool to remove any remaining debris.

If the tank is old and has a build-up of grime, take this opportunity to give it a wipe down with plain tap water. But this may require emptying out all the water and then replacing it later with aged tap water.

Put in the new substrate and turn the filter back on. Put the fish back in along with the water they were in.

5. Observe and test

By changing the substrate, you’ve disrupted the nitrogen cycle by removing some of the nitrifying bacteria that was in the substrate.

So you need to watch for spikes of ammonia and nitrites. Test the water after three days.

If the ammonia levels are high, do a 25% water change and test again after three days. If ammonia levels are still high, consider using ammonia removers.

You should also consider getting live bacteria to help speed up the cycling process and stabilise the fish tank.

Keep testing the water every few days until conditions are stable (0 ammonia, 0 nitrites, <20PPM nitrates).

Can You Change Substrate In a Fish Tank Without Removing Fish?

Removing fish before changing the substrate is the safest option. It reduces stress to the fish and lowers the risk of exposure to toxic substances when you stir up the old substrate.

That said, most fish can survive a substrate change, you just need to be careful and gentle.

Turn off the filter and any other equipment like a heater. Next, remove some of the water from the tank to make it easier to remove the substrate. Let the water level drop to half.

Before you remove the old substrate, vacuum it to pick up waste and other debris. You can then scoop out the old substrate.

Start with one half of the tank then finish the other half. The water will probably get cloudy during this process. But as long as you don’t take too long, the fish will be okay.

Put in the new substrate (be sure to clean it first), and top up the tank with the water you’d removed.

Turn on the equipment and give the water a few hours to clear. Observe and test the fish tank for the next one week to make sure everything is good.

Here’s a video that explains the process.