How the amount of substrate is calculated
First find the volume of the bed, then multiply by how heavy the material is for its volume:
| Substrate | Density as poured | Notes |
|---|---|---|
| Gravel | 1.5 kg/L (94 lb/cu ft) | Pea gravel, 2–5 mm |
| Sand | 1.6 kg/L (100 lb/cu ft) | Packs tighter than gravel |
| Planted-tank soil | 1.0 kg/L | Sold by the litre, not by weight |
| Crushed coral | 1.2 kg/L (75 lb/cu ft) | Raises pH and hardness |
| Aragonite sand | 1.45 kg/L (90 lb/cu ft) | Marine and African cichlid tanks |
Densities vary by 10% or so between products and grain sizes, so treat the result as "buy this much, plus a little".
Worked example. A 55-gallon tank is 48 × 13 inches. A 2-inch bed is 48 × 13 × 2 = 1,248 cubic inches, or 20.5 litres. At 1.5 kg per litre that is 30.7 kg, about 68 lb of gravel: four 20 lb bags.
A quick check for standard gravel: about 1 to 1.5 lb per US gallon of tank gives a 1–2 inch bed in most standard tanks.
How deep should aquarium substrate be?
| Tank | Depth |
|---|---|
| Fish only, gravel | 4–5 cm (1.5–2 in) |
| Fish only, sand | 2.5–4 cm (1–1.5 in) |
| Rooted plants | 5–8 cm (2–3 in) |
| Planted aquascape, sloped | 3–4 cm at the front rising to 8–10 cm at the back |
| Bare-bottom (breeding, quarantine, some goldfish and discus tanks) | None |
Deep beds of fine sand can go stagnant. Keep plain sand under about 5 cm unless plants are rooting in it, and stir it lightly now and then, or keep burrowing snails.
Sloped beds
Sloping the substrate up towards the back makes a tank look deeper and gives stem plants more to root in. The calculator averages the front and back depths, which gives the exact volume for an even slope.
Common mistakes
- Not rinsing gravel and sand. Dust clouds the water for days. Rinse in a bucket until the water runs clear. Planted-tank soil is the exception: never rinse it.
- Pouring water straight onto the bed. Lay a plate or plastic bag on the substrate and pour onto that.
- Sharp gravel with bottom dwellers. Corydoras and loaches need smooth sand or rounded gravel to protect their barbels.
- Crushed coral in a soft-water tank. It steadily raises pH and hardness, which tetras, rasboras and most plants dislike.
Questions and answers
How much gravel do I need for my aquarium?
About 1 to 1.5 lb of gravel per US gallon for a 1–2 inch bed. A 20-gallon tank needs 20–30 lb; a 55-gallon tank needs around 60–70 lb for 2 inches. Enter your exact dimensions above for a precise figure.
How much sand do I need for a 55 gallon tank?
For a 1.5-inch bed in a standard 48 × 13 inch 55-gallon tank you need about 54 lb (25 kg) of sand.
How many litres of aqua soil do I need?
Multiply the length and width of the tank in centimetres by the average depth, then divide by 1,000. A 60 × 30 cm tank with a bed sloping from 3 cm to 8 cm needs about 10 litres: one 9-litre bag is slightly short, so buy a small extra bag.
Is sand or gravel better for a fish tank?
Both work. Gravel is easier to vacuum and suits most community fish. Sand is better for corydoras, loaches and other fish that sift or burrow, and waste stays on top where the filter can catch it.
Does substrate reduce the water volume?
Yes. Every litre of gravel displaces roughly 0.6 litres of water. Our volume calculator takes this into account.
Related guides
Sources
- Bulk densities are typical values for dry, loosely poured material and vary by product.