Short answer: For a first tank, use 1 to 2 pounds of inert gravel or aquarium sand per gallon, about a 1 to 2 inch bed. Gravel is the easy all-rounder, sand suits bottom-dwellers like corydoras, and nutrient-rich aquasoil is the planted-tank choice that comes with more upkeep. The substrate is not just decoration: it holds beneficial bacteria, anchors plant roots, and shapes how your fish behave.
Pick it for the fish and plants you want, because changing it later means tearing the tank down. Get the choice right once and it lasts the life of the setup.
What the substrate does
Three jobs, in order of how much they matter. It gives nitrifying bacteria a vast surface to colonize, so the bed is part of your biological filter, not only the floor. It anchors plant roots and, in the case of active soils, feeds them. And it sets the stage for the fish: corydoras sift it for food, kuhli loaches burrow into it, and a dark bed brings out colors that a bright gravel washes out. A bare-bottom tank works for a hospital setup, but a planted community wants a real substrate.
Gravel: the easy all-rounder
Inert aquarium gravel is where most tanks should start. It is cheap, it never clouds the water once rinsed, and waste settles on top where a siphon pulls it out cleanly. It holds rooted plants well enough if you push a root tab into it near heavy feeders, since the gravel itself carries no nutrients. Choose a grain in the 2 to 5 mm range: fine enough that food does not vanish into the gaps, coarse enough that it does not compact. Skip the painted neon gravel, which can flake over time.
Sand: best for bottom-dwellers
Sand gives the most natural look and it is the right call if you keep fish that sift the floor. Corydoras, kuhli loaches, and many catfish graze sand the way they would in a river, and a coarse gravel can wear down their barbels over time. The tradeoff is cleaning: waste sits on the surface rather than sinking in, so you hover the siphon just above it instead of digging in. The guide to cleaning a fish tank covers that technique, which keeps the sand in the tank and the mess in the bucket. Use pool-filter sand or a product sold for aquariums, and rinse it hard before it goes in or the tank clouds for days.
Aquasoil: the planted-tank specialist
Active aquasoil is packed with nutrients and built for growing demanding plants from the roots up. It feeds a carpet or a wall of stems without constant dosing, and it gently lowers pH and softens the water, which suits shrimp and soft-water fish. The costs are real: it is expensive, it leaches ammonia for the first few weeks so the tank must cycle before fish go in, and it breaks down into mush after a year or two and eventually needs replacing. Treat it as a deliberate choice for a planted tank, not a default.
A popular middle path caps a thin layer of aquasoil or a nutrient base with gravel or sand on top. You get the root feeding without the soil clouding the water or washing loose when you plant. It takes more care to set up, so plan the layers before you add water.
How deep, and how much to buy
A 1 to 2 inch bed suits most tanks. Go to the shallow end for a fish-focused tank with few plants, and the deeper end where rooted plants need room to spread, since a carpet or sword wants 2 to 3 inches to hold. Deeper than that in fine sand traps waste and can turn anaerobic, so keep sand beds on the shallow side or stir them occasionally. Work out the exact weight for your tank footprint and depth with the aquarium substrate calculator, or read off the pounds for a standard tank from our gravel and sand quantity chart, so you buy once instead of running back to the store mid-setup.
Matching substrate to your tank
Sort it by what you are building. A low-tech planted community does well on inert gravel with root tabs under the hungry plants, which is the same low-effort path as keeping easy low-light plants. A tank built around corydoras or loaches wants smooth sand. A high-tech aquascape chasing a lush carpet earns its aquasoil. Once the bed is down, planting the tank properly, roots in and crown above the surface, is what decides whether those plants take hold.
Aquarium substrate FAQ
How much substrate do I need for my tank?
Plan on 1 to 2 pounds per gallon for a 1 to 2 inch bed, more for a deep planted layout. The exact figure depends on the tank footprint and the depth you want, which the substrate calculator works out from your measurements.
Can I use play sand or pool sand in an aquarium?
Pool-filter sand is a safe, cheap choice and a hobby favorite. Play sand works too but is finer and dustier, so it needs far more rinsing and compacts more readily. Rinse either one until the runoff is clear before it goes in.
Do I need special substrate for live plants?
Not for easy low-light plants, which grow in plain gravel with a root tab tucked in nearby. Demanding carpets and heavy root feeders do far better in a nutrient-rich aquasoil, or on an inert bed with a steady fertilizer routine.
Can I change substrate in an established tank?
You can, but it disturbs the bacteria living in the bed, so do it in stages and watch your water afterward. Swap a section at a time over a few weeks rather than stripping it all at once, which protects the cycle and spares the fish an ammonia spike.
