The substrate formula
Substrate is sold by weight or by volume, but the tank floor only tells you volume. So the calculation has two steps: volume from the floor and depth, then weight from the bulk density, which is the weight of a litre of loose material including the gaps between grains.
Average depth = (front depth + back depth) ÷ 2
Volume (L) = length (cm) × width (cm) × average depth (cm) ÷ 1,000
Weight (kg) = volume (L) × bulk density (kg/L)
How much substrate common tanks need
| Inside floor | Depth | Volume | Gravel (1.5 kg/L) | Sand (1.6 kg/L) |
|---|---|---|---|---|
| 60 × 30 cm | 5 cm | 9 L | 13.5 kg (29.8 lb) | 14.4 kg (31.7 lb) |
| 60 × 30 cm | 7.5 cm | 13.5 L | 20.3 kg (44.6 lb) | 21.6 kg (47.6 lb) |
| 80 × 35 cm | 5 cm | 14 L | 21 kg (46.3 lb) | 22.4 kg (49.4 lb) |
| 90 × 45 cm | 5 cm | 20.3 L | 30.4 kg (67 lb) | 32.4 kg (71.4 lb) |
| 120 × 45 cm | 5 cm | 27 L | 40.5 kg (89.3 lb) | 43.2 kg (95.2 lb) |
| 20 gal high (24 × 12 in) | 5 cm (2 in) | 9.3 L | 13.9 kg (30.7 lb) | 14.9 kg (32.8 lb) |
| 29 gal (30 × 12 in) | 5 cm (2 in) | 11.6 L | 17.4 kg (38.4 lb) | 18.6 kg (41 lb) |
| 55 gal (48 × 13 in) | 5 cm (2 in) | 20.1 L | 30.2 kg (66.6 lb) | 32.2 kg (71 lb) |
Worked example: sloped aquasoil bed
A planted tank has an inside floor of 90 × 45 cm. The bed slopes from 3 cm at the front to 8 cm at the back, so the average depth is 5.5 cm.
Volume = 90 × 45 × 5.5 ÷ 1,000 = 22.3 litres. Aquasoil is sold by volume, so with 9 litre bags you need 22.3 ÷ 9 = 2.5, rounded up to 3 bags. The same bed in gravel would weigh about 33 kg.
Choosing depth and type
- Rooted plants generally do better in a deeper bed, often 5–8 cm (2–3 in), with the deepest part at the back.
- Sand suits bottom-dwellers such as corydoras and loaches that sift through it. Keep fine sand fairly shallow so it does not compact into airless pockets.
- Crushed coral and aragonite slowly raise hardness and pH: useful for African cichlids and marine tanks, wrong for soft-water fish.
- Rinse gravel and sand before use. Most aquasoils should not be rinsed; follow the bag.
- The old “1 lb per gallon” rule ignores tank shape. A 29 gallon tank (30 × 12 in floor) needs only about 19.5 lb of gravel for 1 inch, so measuring the floor is more reliable.
