There is a huge range of substrates available and they can play many more roles than just decorative.
Fish feel more settled when swimming over substrates and sand and gravel will coat with beneficial nitrifying bacteria over time, aiding water quality and biological filtration.
Different substrates can help to set a theme like a coral reef, soft water backwater or stream, and aragonite sand, silver sand or large, round gravel could be used respectively to help create the look and feel of the natural environment for the fish which come from those habitats in nature.
Coloured gravels are fun for children and aquarium soils or planted tank substrates are perfect for aquascaping planted tanks and plant roots.
Some need to be pH neutral and inert substrate whereas others can be used to raise pH for those fish that require it. So no matter what style of fish tank you want to achieve, there’s a substrate to suit.
What can you use for fish tank substrate?
Aquarium substrate must sink, not cloud the water, and for most fish, not affect the pH of the water. It must also be made from a material that beneficial bacteria can colonize. That’s why you should only use aquatic substrates from a local fish store as for most of them, all of the above applies.
Not all sands and gravels are inert substrates, as many from the construction industry contain lime, which raises pH and hardness of the water, and will be very dirty, clouding the water.
The most common aquarium substrate is gravel between 3mm and 10mm in diameter.
Aquarium gravel is easy to keep clean, easy to plant into and if it says it’s inert, it won’t raise the pH of aquarium water, making it perfect for most coldwater and tropicals.
Gravel materials come in all sorts of shapes, colours and sizes from angular chips to rounded grains and in every colour from natural beige, brown, and grey to blue, black, and even bright pink.
Most people choose natural-looking gravels to complement rocks, live plants and driftwood but coloured gravels are also widely used to create lively underwater scenes with plastic plants, and ornaments. It’s down to personal preference with most substrates.
Is substrate necessary for fish tanks?
An aquarium substrate isn’t absolutely necessary and some aquatic stores display fish for sale over bare-bottomed tanks.
Substrate serves to aid algae growth, trapping fish waste, uneaten food and certain parasites, so that’s why some people choose not to add it to their tanks.
You can run a cleaner tank without it, filters pick up more dirt particles and the substrate dwelling stages of some parasites may be hindered.
That’s why hospital tanks and quarantine tanks shouldn’t have a substrate either.
But many fish species have adapted to use substrates in their natural habitat to feed, breed, camouflage or even dive into to escape predators.
For those fish species, a substrate can provide environmental enrichment and lower their stress levels.
Sand substrates are good for Corydoras catfish, for whom it’s in their nature to sift the sand for food particles. By doing this they’ll help to keep it clean too.
So substrate provides decoration but can also be important for some species. If you want to create a planted tank they’ll need substrate to root into, and 5cm or deeper is best for plant roots. Add root tabs to natural gravel for extra key nutrients.
Large grain gravel can be used for egg scattering species.
For best results with live plants use planted tank substrates like aquascaping soils which are soft to encourage root growth but also provide vital fertiliser for plants. Most plants are rooted plants, but you can even get live aquarium plants that don’t need any substrate material.
If aquarium substrate is out in your tank but you still want to grow plants, choose Anubias, Java fern or moss tied to wood, or opt for floating plants like Salvinia.
Can I use dirt as an aquarium substrate?
There is a type of planted aquarium known as a Walstad Method tank, named after ecologist Diana Walstad.
She documented her success using ordinary garden soil material in the planted tank although our garden soils differ greatly in composition and suitability for aquariums so are best left out. They may cloud the water, clog filters, cause algae to grow and alter water chemistry.
Instead, use a planted tank substrate like Tropica aquarium soil instead for the best possible plant growth, as it has high cation exchange capacity and proven planted aquarium results around the world.
Dirt may seem like a no-cost solution to planted substrates but if it goes wrong you’ll have to strip the aquarium down and start again. You may experience an algae bloom if that terrestrial dirt dumps ammonia once submerged in water.
What is the difference between aquarium gravel and substrate?
Substrate is the term the aquarium community give to any bottom layer added to a fish tank. This could be sand, soil, or gravel, and come in a myriad of shapes and sizes.
Aquarium gravels are a type of inert substrates and the most commonly used in our fish tanks. Newly purchased gravel should be rinsed in tapwater to remove dust and then placed into an empty tank to a depth of around 5cm.
Gravel can be sloped up at the back to provide perspective and to encourage detritus to settle at the front of the tank where it can be easily removed with a gravel vacuum.
What colour substrate should I choose?
Fish feel more comfortable and display better colouration over a dark substrate. Black can be particularly effective in a tropical tank for enhancing colouration.
For biotope tanks use natural sands and gravels like you would see in nature. Use fine substrates for still, backwaters and large pebbles for fast-flowing stream tanks.
The same can apply to coloured sands and gravels. Fish may appear more colourful over a dark blue gravel substrate than over a pale yellow, white, or pink substrate material for example.
But if colour is your fish tank thing try a black aquarium background, blue gravel, green plants, and red and yellow fish like goldfish or platies.
Light-coloured substrates grow more algae and coloured substrates fade over time, so replace a proportion, avoid excessive light and employ algae grazers to keep those substrates looking at their best and prevent algae growth.
Which substrate do I need for my marine tank?
Part of the appeal of a marine aquarium is the bright sand base that looks like a beach on a tropical island. Those white tropical sands are actually coral sand, tiny fragments of long-dead coral skeletons, crushed coral and shells which have been worn down into sand-sized granules.
It’s coral sand that you need for your marine aquarium water chemistry too, not normal beach or playpit sand as that contains silicates which cause algae. Coral sand substrate has more uses than just aesthetics.
Being lime-based it releases buffers that help to raise the pH - something that’s important in marine aquaria, and one of the reasons why it shouldn’t be used in standard freshwater aquaria.
It also provides a home for a myriad of worms, copepods and amphipods which in turn feed wrasses and dragonettes.
Some marine fish need a bed of sand as they are adapted to sift and find food within it, or even to sleep in and hide away from would-be predators at night.
What aquarium sand is available?
Coral sand varies in size, texture and colour. You can get fine, medium, coarse, or a mixture of grain sizes for a natural look. You can even get black. Aragonite is the best type of sand to buy as it has the best buffering capacity, and is available as normal, dry sand or “live.”
Live sand is aragonite sand that contains moisture and with it, colonies of live beneficial bacteria. The manufacturers claim that it can help to cycle a new marine tank by seeding the tank with millions of naturally occurring microbes, so if you have a new reef tank and want it to mature and become its own functioning ecosystem, live sand can help with that and water chemistry.
How much sand do I need for my tank?
There is a balance between having too much sand and too little. You need enough to cover the bottom pane of glass in the aquarium, plus a bit extra so that the base isn’t exposed if the fish start to dig. A layer of 1-3cm is about right. Place the rocks onto the base first and then sprinkle sand around them, so the rocks can’t be undermined and fall over.
Gravel around 5cm/2” deep can start to harbour uneaten food and are blamed for excess nitrates, so thin it out or vacuum regularly to prevent dead spots and anaerobic areas in the substrate.
A sand layer of 10cm depth is known as a deep sand bed, and that can be so deep that it can actually aid water quality by reducing nitrates, but although popular over twenty years ago there are now lots of other ways to control nitrates in liquid form.
Vacuum your sand bed regularly to prevent the build-up of dead organic matter. Livestock can help too, like Sand sifting starfish, Nassarius snails, Chalk gobies and Orange spot gobies.
Due to the nature of Aragonite, it dissolves over time, so by adding more Aragonite sand every year you get fresh, clean white sand but also increased buffering capacity.
The golden rules of fish tank substrates
Work out how much gravel you need using our calculator. We’ve based it on a 5cm deep layer across the base of your aquarium
Rinse all-new inert substrates in tapwater, agitating the grains vigorously until the water runs clear.
Don’t rinse aquascaping soil or live sand. You risk washing it and its live bacteria away and losing your added biological filtration.
Clean fish tank gravel regularly using a gravel vacuum. Then use that dirty tank water to clean your filter sponges.
Avoid sharp substrates with bottom-dwelling species like catfish and loach species. It may damage their delicate barbles.
Never add live sand to unprepared aquarium water. Water should be dechlorinated and brought to temperature so the bacteria is intact. For saltwater live sand, salinity must also be at 1.024-1.025 s.g
Only use calcareous substrates for Rift Lake cichlids, marines and other fish that actually need hard water, like Mollies for example.
Should I swirl the sand?
Running your fingers through the substrate is good practice as it will dislodge detritus and uneaten food and help to keep the good bacteria aerated.
But it’s better to hoover it with a gravel vacuum. It cleans the gravel, removing nutrients and does a partial water change at the same time, which is great for the long term health of your aquarium.
If you don’t want to take water out while cleaning, get a battery-powered gravel vac that traps the waste and recycles the aquarium water.
What are the dark patches in my sand?
Dark patches occur when sand turns anaerobic. These anaerobic zones form under stones and ornaments - especially if uneaten foods gets under there, and if you disturb them you’ll smell rotten eggs. It can happen in new and old substrate.
Anaerobic substrates are bad for aquariums and rooted plants, another reason to have a fine sand layer, (as deep sand makes it worse,) and to clean sand regularly.
Is slime algae on the substrate Cyanobacteria?
Cyanobacteria is commonly referred to as slime algae but as the name suggests, it is actually a bacteria, not algae.
It behaves like algae, spreading over glass and decoration. Cyanobacteria is green colour in freshwater tanks and red in saltwater setups.
It starts on the substrate, especially dirty substrates as it uses detritus and other waste nutrients as a food source. It then starts to spread up the tank glass from the bottom.
Syphon out as much of the slime as you can on a daily basis until you get on top of it.
In severe cases thin out the substrate layer to less than 1cm or remove the substrate entirely.
Increase water changes, flow, aeration and mechanical filtration so that more free-floating particles are captured. If no live plants or corals are present leave the tank lights off for a week as Cyanobacteria needs light. The same rules can be applied to prevent algae growth.
Can I mix aquarium substrates?
Yes, you can mix substrates to provide extra colour or an even more natural environment. Mix coloured gravels to make up your own unique fish tank blend.
Add larger crushed coral gravel and shells to reef sand to make a rubble zone on the tank bottom that’s perfect for Shrimp and Goby pairs to build their homes.
Aquascapers use a mix of chemically inert substrate sizes and styles to create different areas at the front of their planted tanks.
If you want to keep other substrates apart in most tanks, you won’t be able to do it with fish that dig, or if you place fine substrates like sand on top of coarse substrates like gravel. The sand will work its way to the bottom.
Coral gravel can be mixed with inert substrates to raise pH in Rift Lake Cichlid aquariums.
Corydoras mess up my planted soil substrate. Is there anything I can use instead?
Caribsea Eco Complete has a harder grain than soil so won’t disintegrate if your catfish sift and dig into it. Or replace with new soil with a harder grain.
Or use substrate fertiliser as a lower layer and a common substrate on top. Peat is present in some substrate fertiliser and soil which softens water and lowers pH levels.
You could use plain gravel with root tabs inserted into it or Caribsea Eco Complete exclusively as it’s a great substrate and a complete substrate that already contains nutrients.