Guide
How deep should aquarium substrate be?
By the Aquarium Calculator team
For most aquariums, 5 cm (2 inches) of substrate is the right depth. Sand works best at 2.5–5 cm (1–2 in), gravel at about 5 cm (2 in), and planted tanks at 5–8 cm (2–3 in), usually sloped deeper at the back. For a 60 × 30 cm tank, a 5 cm bed means about 14 kg of sand or 7 kg of aquasoil.
Recommended depths at a glance
Sand for fish-only or community tanks: 2.5–5 cm (1–2 in). Fine sand packs tightly, so a shallow bed stays clean and healthy. Gravel: about 5 cm (2 in), enough to anchor decorations and hold beneficial bacteria. Planted tanks with aquasoil or nutrient-rich substrate: 5–8 cm (2–3 in), often sloped from about 3 cm at the front to 8 cm at the back. Bare-bottom tanks, used for breeding, quarantine or some large fish, have no substrate at all.
Bottom-dwellers change the picture. Corydoras catfish and many loaches sift through the bed with sensitive barbels, so they need smooth sand, and burrowing species such as kuhli loaches appreciate a slightly deeper sand bed they can dig into.
Why depth matters
Too shallow and plants cannot root, decorations topple, and there is little surface for bacteria. Too deep, especially with fine sand, and the lower layers can lose oxygen. Pockets of stagnant, oxygen-free substrate can produce hydrogen sulphide gas, which smells of rotten eggs when disturbed. Snails such as Malaysian trumpet snails, burrowing fish and a gentle stir during water changes keep deeper sand beds healthy.
Depth also changes how much water your tank holds. A 5 cm bed in a 100 × 40 cm tank takes up 20 litres, a tenth of a 200 L tank. That is why medication and fertiliser doses should be based on net volume, which the volume calculator works out when you enter your substrate depth.
Planted tanks: depth, layers and slopes
Most rooted plants do well in 5–8 cm of substrate; heavy root feeders such as Amazon swords and cryptocorynes appreciate the deeper end. Many aquascapers use a sloped bed, shallow at the front and deep at the back, which looks deeper and gives large background plants more root room. Aquasoil is the simplest choice for planted tanks because it contains nutrients and softens the water slightly; it is also much lighter than sand, at about 0.8 kg per litre.
If you prefer inert sand or gravel, add root tabs under heavy feeders, or use a thin nutrient layer under a cap of 3–5 cm of sand or gravel. Keep the cap deep enough that the nutrient layer does not escape into the water when you replant.
How much substrate to buy
Multiply the base length by the width and the depth to get the bed volume, then multiply by the material's density. At 5 cm deep, a 60 × 30 cm tank (about 60 L) needs about 9 litres of substrate: 14.4 kg of sand, 13.5 kg of gravel or 7.2 kg of aquasoil. An 80 × 35 cm tank (about 100 L) needs 14 litres: 22.4 kg of sand, 21 kg of gravel or 11.2 kg of aquasoil. A 100 × 40 cm tank (200 L) needs 20 litres: 32 kg of sand, 30 kg of gravel or 16 kg of aquasoil.
For a sloped bed, use the average depth: a slope from 3 cm at the front to 8 cm at the back averages 5.5 cm. Buy around 10% extra to allow for settling and shaping. The substrate calculator gives the exact weight and volume for any base size, depth and material, and how much water the bed will displace.
Preparing and adding substrate
Rinse sand and gravel in a bucket until the water runs almost clear, otherwise fine dust clouds the tank for days. Do not rinse aquasoil, which breaks down when handled roughly; add it dry and fill the tank slowly over a plate or bag so the water does not dig holes in the bed.
Add the substrate before the hardscape where rocks need to sit on the glass, or place heavy rocks first and fill around them so burrowing fish cannot undermine them. Then add water slowly, plant, and let the filter clear any remaining haze.
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