Buying substrate is surprisingly easy to get wrong. Too little and plants struggle to root and the bed looks thin; too much and you lose water volume, trap waste and pay for bags you didn't need. The most common mistake is using a "pounds per gallon" rule on a tank whose shape doesn't fit it.
The formula
Substrate fills a slab across the bottom of the tank, so you need three numbers: the inside length, the inside width and the depth.
volume = length × width × depthweight = volume × bulk density
In inches, divide cubic inches by 1,728 for cubic feet; gravel weighs about 100 lb per cubic foot. In centimeters, divide cubic centimeters by 1,000 for liters; gravel weighs about 1.6 kg per liter. For a sloped bed, use the average of the front and back depths.
Example: a 20-gallon long tank has an inside footprint of about 30 × 12 inches. A 2-inch bed is 720 cubic inches, or 0.42 cubic feet, which weighs about 42 lb of gravel. A pound-per-gallon rule would have said 40 lb — close, because this tank has typical proportions.
Why the "1 lb per gallon per inch" rule fails
The rule assumes a tank of average proportions. Tall tanks have a small footprint for their gallons, and shallow tanks a large one. A 20-gallon high (24 × 12 inches) and a 20-gallon long (30 × 12 inches) hold almost the same water, but the long tank has 25% more floor and needs 25% more gravel. A 40-gallon breeder needs more gravel than a 55, because its 36 × 18 inch floor is larger than the 55's 48 × 13. Always calculate from the footprint.
Substrate needed for standard tanks
| Tank | 1 inch | 2 inches | 3 inches |
|---|---|---|---|
| 5 gallon | 7 lb | 14 lb | 21 lb |
| 10 gallon | 11 lb | 22 lb | 33 lb |
| 15 gallon | 16 lb | 32 lb | 48 lb |
| 20 gallon high | 16 lb | 32 lb | 48 lb |
| 20 gallon long | 20 lb | 40 lb | 60 lb |
| 29 gallon | 20 lb | 40 lb | 60 lb |
| 40 gallon breeder | 36 lb | 72 lb | 108 lb |
| 40 gallon long | 32 lb | 63 lb | 95 lb |
| 55 gallon | 34 lb | 69 lb | 103 lb |
| 65 gallon | 35 lb | 70 lb | 105 lb |
| 75 gallon | 47 lb | 94 lb | 141 lb |
| 90 gallon | 47 lb | 94 lb | 141 lb |
| 125 gallon | 71 lb | 142 lb | 213 lb |
| 150 gallon | 70 lb | 140 lb | 209 lb |
Weights are for aquarium gravel over the inside footprint; sand is about 3% lighter and aragonite about 10% lighter.
How deep should the bed be?
- Fish-only community: 1–2 inches. Enough to anchor decor and look natural, shallow enough to vacuum easily.
- Planted tanks: 2–3 inches on average, often sloping from 1.5 inches at the front to 3–4 inches at the back. Root feeders such as Amazon swords and cryptocorynes appreciate the depth.
- Bottom dwellers: corydoras, kuhli loaches and many plecos sift and burrow; give them 1.5–2 inches of smooth, fine sand.
- Reef and marine: either a shallow sand bed of 0.5–1.5 inches, easy to keep clean, or a deep sand bed of 4 inches or more meant to host anaerobic bacteria that reduce nitrate. Avoid the 2–3 inch middle ground, which traps detritus without the benefit.
- Bare bottom: common for goldfish grow-out, breeding and hospital tanks, because waste is easy to siphon.
Choosing the type
| Substrate | Bulk density | Best for |
|---|---|---|
| Aquarium gravel | 1.6 kg/L | General community tanks; easy to vacuum |
| Sand (silica, pool filter) | 1.55 kg/L | Corydoras, loaches, natural looks; cheap in bulk |
| Aragonite | 1.45 kg/L | Reefs and African cichlids; buffers pH and hardness upward |
| Aqua soil | ≈ 1.0 kg/L | Heavily planted tanks; softens water, lowers pH |
Aqua soils are sold in liters, not pounds, so calculate the volume and compare it with the bag. They break down over 1–3 years and are usually capped or replaced rather than vacuumed deeply. Aragonite will raise pH and hardness, which suits African cichlids and marine tanks but not soft-water fish such as rams or cardinal tetras.
Sloping the bed
A slope from front to back adds depth to an aquascape and puts the deepest soil where the largest plants grow. Calculate it with the average depth: 1.5 inches at the front and 3.5 at the back is the same amount of substrate as a flat 2.5-inch bed. Sand slowly flattens over months; rocks or hardscape placed as low terraces keep a slope in place.
Buying and rinsing
- Round up to whole bags and buy one extra: some is lost rinsing, and slopes always use more than planned.
- Rinse gravel and sand in a bucket until the water runs mostly clear. Do not rinse aqua soil — it washes the nutrients out and turns to mud.
- Pour water onto a plate or bag when filling a sand tank to avoid craters and clouding.
- Remember that substrate takes water volume: a 2-inch bed in a 55 displaces about 5 gallons. Use the volume calculator with substrate depth to dose correctly.
Ready to buy? The substrate calculator handles sloped beds, four substrate types and the number of bags for any bag size.