Choosing Aquarium Substrate

Substrate is more than decoration — it affects plants, bottom-dwellers and even water chemistry. A few simple rules keep the choice easy.

The layer on the tank floor does several jobs: it anchors plants, gives bottom-dwellers somewhere to sift, and can even influence water chemistry. For most tanks, one type suits one job.

The main types

Substrate	Good for	Notes
Smooth sand	Bottom-dwellers (corys, loaches), look	Gentle on barbels; can compact
Rounded gravel	General community tanks	Avoid sharp edges
Planting soil	Rooted plants	Boosts nutrients; can leach ammonia at first
Aragonite/coral	Hard-water and African-cichlid tanks	Raises GH/KH

A note on bottom-dwellers

Corydoras and loaches have sensitive barbels. Go for smooth, rounded sand for them — rough or sharp gravel wears the barbels and can lead to infection. This is one of the most common substrate mistakes.

How much?

Around 3–5 cm deep suits most tanks — deep enough for plants, shallow enough to clean easily. Very deep beds can compact and trap gas, especially with sand.

If you keep plants

Easy plants like anubias and java fern grow on wood or rock and do not need rich soil. Rooted plants benefit from a planting soil or root tabs. Do not mix heavy soils with a sifting bottom-dweller — the fish will stir everything up.

How deep should it be?

Three to five centimetres suits most tanks — deep enough to anchor plants, shallow enough to vacuum without turning the whole bed over. Very deep beds, especially of fine sand, can compact and trap gas pockets, and they hide a lot of decaying waste. If you keep rooted plants, a slightly deeper layer at the back is fine, but keep the front shallow so you can see and clean it.

For a sifting bottom-dweller like a cory or a loach, a shallower, softer bed is kinder: they grub through it constantly, and a deep rough layer abrades their barbels and their gills.

Substrate for plants

Not every plant needs soil. Epiphytes like anubias, java fern and moss grow attached to wood or rock and take nutrients from the water, so they are happy on any substrate — and their roots rot if buried. Rooted plants such as cryptocorynes and swords do better in a nutrient-rich layer, whether that is a dedicated planting soil or ordinary gravel boosted with root tabs pushed down to the roots. Do not put planting soil under a fish that constantly digs, or it will stir the whole thing into the water column.

Colour and the look of the tank

Substrate sets the mood more than most equipment. Pale sand makes a tank look bright and open and shows off a fish's colour, but it shows every speck of dirt. Dark gravel or sand makes colours pop, hides mulm and suits a moody, planted look. Bright, unnatural gravel can cheapen an otherwise lovely tank. Think about the fish and plants you keep, and pick a tone you are happy to look at every day.

Cleaning different substrates

Gravel is easy: push the siphon tube into it and the trapped waste lifts out. Sand is slower — vacuum just above the surface rather than plunging in, and stir the top layer now and then so it does not go anoxic. Never strip a whole bed at once; clean a section each week on rotation, leaving the bacteria and the plants undisturbed.

How much to buy

As a rough guide, you need about 1 kg of gravel per litre of tank volume to reach a 4-5 cm bed, or a little more for sand, which packs denser. Buying slightly more than you think lets you build a gentle slope — deeper at the back, shallower at the front — which looks natural and gives rooted plants room while keeping the front easy to clean.

Substrate and the first weeks

New substrate should be rinsed until the water runs clear before it goes in, or the tank will cloud for days. Planting soil often releases a little ammonia at first, so expect to test daily and change water in the first week or two if you use it. Gravel and sand are inert and do not affect the cycle. Let the dust clear before adding fish, and do not disturb the whole bed once the tank is running.

Changing substrate later

Swapping substrate in an established tank is disruptive, so it is worth getting right the first time. If you must change it, do it in halves on different days so the bacteria in the bed and filter are not lost at once, and keep the filter running throughout. Expect a possible mini-cycle and test for a week or two.

Is sand or gravel better?

Sand suits bottom-dwellers and looks natural; gravel suits general tanks. Both work — match the fish.

Can substrate change my water?

Yes — aragonite and crushed coral raise GH/KH, while some soils lower pH at first.

Do I need to rinse it?

Yes, rinse gravel or sand until the water runs clear before adding it.

Tools for this guide

Tank weight calculator

Sources and research notes

Guidance follows standard fishkeeping practice; smooth substrate protects the barbels of bottom-dwelling fish.

Practical Fishkeeping

Written and edited by the Tankwise Editorial. Every guide answers one real fishkeeping question, is checked against the sources listed above, and is updated when guidance changes. We are an independent editorial desk, not veterinary or aquarium professionals — read how we research and update our guides.