How to Choose Aquarium Substrate for a Planted Tank

Substrate is the one part of a planted tank that is hard to change later. Once plants are rooted and animals are settled, swapping it means a full teardown. A few minutes of planning now saves a lot of work.

Aquasoil, gravel, or sand: which substrate suits a planted tank?

Aquasoil

Aquasoils are baked granules made from soil, often of volcanic or similar mineral origin, designed for planted tanks. Their main advantages are:

• Nutrients for roots. Many contain nutrients at the start, which helps rooted plants establish.
• Softer water. Tropica states that its Aquarium Soil lowers pH and slightly affects water chemistry. Many aquasoils work this way, which suits many plants and soft-water animals.
• Light, rounded granules that roots penetrate easily. Tropica lists a 2-3 mm grain size for its Aquarium Soil.

The trade-offs:

• Early ammonia. Many new aquasoils release ammonia or ammonium into the water for the first weeks. Aqua Design Amano's guidance for its soil notes that ammonia is generally present right after setup, and that it becomes more toxic to fish and shrimp if pH rises above 7.
• Frequent early water changes. Tropica recommends changing 25 to 50% of the water at least twice a week during the first 4 weeks with its soil.
• Finite lifespan. Granules gradually break down and nutrients are used up over time, after which root tabs become useful.
• Cloudiness when disturbed. Digging and replanting can stir up fine particles.

Inert gravel

Inert gravel does not change water chemistry or supply nutrients. It is stable, long-lasting, and easy to clean. Tropica's setup guide suggests a grain size of roughly 0.8 to 4 mm for good water circulation around roots. Smaller grains hold plants more easily than coarse pebbles.

In plain gravel, epiphytes and water-column feeders do well. Heavy root feeders such as cryptocorynes and Amazon swords usually need root tabs.

Sand

Fine sand gives a clean, natural look and suits fish that sift the bottom. It is inert, like gravel, so root feeders again need root tabs. Very fine sand can compact over time, and some keepers prefer a slightly coarser grade for planted areas. Sand also tends to settle on top of other substrates, so it is usually kept to separate areas or used as a top layer deliberately.

Capping and layered beds

A middle route is a nutrient layer under an inert cap. Tropica's setup guide describes a thin nutrient layer of about half a centimetre to 1 cm, covered by gravel, with the nutrient layer concentrated where strong-rooted plants will go. The cap keeps the nutrient layer in place and reduces clouding.

Capped beds work well if you rarely uproot plants. Frequent replanting can bring the lower layer to the surface.

Root tabs

Root tabs are fertilizer capsules or tablets pushed into the substrate near root-feeding plants. They let you grow crypts, swords, and similar plants in inert substrates, and they extend the useful life of an older aquasoil. Push them deep enough that they stay buried, and follow the product's directions for spacing and frequency. Planted tank fertilizer basics explains how root tabs fit alongside liquid fertilizers.

Does aquasoil affect cycling?

If you use an aquasoil that releases ammonia, that ammonia can help start the nitrogen cycle, but it also means the tank is not safe for animals until tests confirm otherwise. Test ammonia and nitrite regularly, keep up the water changes the manufacturer recommends, and add livestock only when both read zero. Inert gravel and sand release nothing, so you will need to supply an ammonia source to cycle. The full process is in aquarium cycling and water testing.

Effect on pH and KH

Many aquasoils absorb carbonate hardness (KH) and lower pH, especially when new. That can be helpful for soft-water fish and shrimp, but it also means pH may be less stable in the early months. Inert gravel and sand are usually neutral, but some gravels contain limestone or shell, which raises hardness and pH. If you are unsure, a simple check is to drop a little vinegar on a dry sample; fizzing suggests carbonate content.

Test pH, KH, and GH before and after setup so you know where your water is heading. The test kits hub covers what to measure.

Matching substrate to livestock

• Corydoras and other bottom sifters. Care guidance commonly recommends soft sand or smooth, fine gravel to protect their sensitive barbels. Avoid sharp or coarse grains in their areas.
• Shrimp. Many shrimp keepers choose aquasoil because it softens water and lowers pH, which suits soft-water species. Check the needs of the specific shrimp you plan to keep, since some prefer harder water.
• Digging fish. Fish that excavate will move light aquasoil and fine sand around. Heavier gravel and well-anchored plants hold up better.
• Hard-water fish. Livebearers and some cichlids generally prefer harder, more alkaline water, which a softening aquasoil works against.

How deep should planted tank substrate be?

Rooted plants need enough depth to anchor and spread. Tropica's guidance is to slope the substrate upward toward the back, with at least 3 to 4 cm at the front and 6 to 8 cm at the back. A slope gives the layout a sense of depth and puts taller, root-heavy plants where the substrate is deepest. Epiphyte-only tanks can use a much thinner layer.

Very deep beds in areas with no roots can trap debris, so keep depth where plants will actually use it.

How much substrate do I need?

Work it out by volume rather than by guessing from bag sizes. This is the same formula Tropica gives for its soil: length × width × depth in centimetres, divided by 1,000, equals litres.

1. Measure the inside length and width of the tank.
2. Decide your average depth. For a slope, average the front and back depths.
3. Multiply length by width by average depth.

In centimetres, divide the result by 1,000 to get litres. For example, a 60 by 30 cm footprint with a slope from 4 cm to 8 cm has an average depth of 6 cm: 60 × 30 × 6 = 10,800 cubic centimetres, or about 10.8 litres. In inches, multiply length, width, and average depth, then divide by about 61 to convert cubic inches to litres. Worked the same way, a 20 gallon long (30 × 12 in footprint) sloped from about 1.5 in at the front to 3 in at the back averages 2.25 in: 30 × 12 × 2.25 = 810 cubic inches, or roughly 13 litres before allowing for hardscape.

Compare that volume with the volume printed on the bag. If the bag lists only weight, check the manufacturer's coverage guidance, since density varies between products. Hardscape takes up space, but many keepers still buy a little extra to allow for settling and slope.

Rinsing

Most inert gravels and sands benefit from rinsing in a bucket until the water runs mostly clear, which reduces cloudiness. Aquasoils are usually a different matter: many manufacturers advise against rinsing because it breaks down granules and washes out nutrients. Always follow the instructions for your specific substrate.

Quick answers

Is aquasoil or gravel better for plants? Aquasoil suits tanks with many rooted plants and soft water, but brings early ammonia and frequent water changes. Inert gravel or sand gives stable chemistry, with root tabs for heavy root feeders.

How deep should substrate be? Tropica recommends sloping it from at least 3-4 cm at the front to 6-8 cm at the back. Epiphyte-only tanks can use a much thinner layer.

Should I rinse aquasoil? Usually not. Many aquasoil makers advise against rinsing because it breaks down the granules, while most inert gravels and sands benefit from it.

When can I add fish to a tank with new aquasoil? Only after ammonia and nitrite both test at zero. Ammonia is more toxic at higher pH, so take extra care with hard or alkaline tap water.

Common mistakes

• Not checking depth before buying. Running short halfway through a scape is common.
• Root feeders in inert substrate with no root tabs.
• Rinsing aquasoil when the maker says not to.
• Adding livestock during the ammonia phase of a new aquasoil.
• Choosing sharp gravel for corydoras.
• Mixing sand over gravel without a barrier, which leads to sand sinking through over time.
• Ignoring hardness effects. A limestone gravel or a softening soil can push water away from what your animals need.

Next steps

• Choose plants for your substrate in easy aquarium plants for beginners.
• Learn how to cycle with or without aquasoil in aquarium cycling and water testing.
• Compare specific aquasoils, clay gravels, and sands for planted tanks or browse the substrates hub, and plan root tabs with the fertilizers hub.
• Weighing an inert clay gravel against an aquasoil? See Seachem Flourite vs UNS Controsoil and our UNS Controsoil review.
• See where substrate fits in the full build with the beginner planted aquarium guide.