How Much Gravel or Sand Does Your Aquarium Need?
Enter your tank base, pick a bed style and a medium, and get the exact amount of gravel or sand to buy — by weight and volume, plus bag count and depth warnings sourced from the aquarium community.
Your substrate amount
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Frequently asked questions
How much substrate do I need for a 20 gallon long tank?
A 20 gallon long has a 30×12 inch base. With a standard sloped bed (2 inches at the front, 3 at the back) of medium-coarse sand, that is about 900 cubic inches — roughly 14.7 liters, or 50 lb. A planted setup runs deeper (3–4 inches) and uses less-dense aquasoil, so always calculate from target depth rather than a flat per-gallon number.
Is the "1 pound per gallon" rule for substrate accurate?
It is a quick rule of thumb for a thin, plant-free bed — about 20 lb for a 20 gallon tank — and it underestimates most real setups. Planted and deeper beds are calculated from target depth times material density, which typically lands well above 1 lb per gallon. This calculator shows both calibers side by side with their conditions instead of picking one for you.
How deep should aquarium substrate be?
Community experience clusters in a 1–4 inch range. Around 1 inch suits a bare, lightly decorated look; 2–3 inches front-to-back is the standard slope; rooted plants want 3–4 inches in the planting zones. Going beyond 4 inches of inert sand raises a different question — see the anaerobic risk note the calculator shows automatically.
Can deep sand go anaerobic?
Inert fine or medium sand at 5 inches or deeper can develop oxygen-free zones that release hydrogen sulfide — a real risk backed by repeated community accidents. The deep sand bed counter-view holds when Malaysian trumpet snails stir the bed, rooting plants oxygenate it, or the grain is coarse. The calculator flags the condition rather than banning deep sand outright.
How do I calculate substrate for a sloped aquascape?
Use the mean depth: volume equals base area times (front depth plus back depth) divided by two — the trapezoid formula. Do not add a 10–15 percent surcharge for the slope; the mean depth already accounts for it. For steep differences of 2 inches or more, build the structure with mesh-bagged lava rock and cap it with your calculated depth of substrate.
Do planted tanks need a special substrate?
Rooted plants benefit from a two-layer bed: about 1 inch of nutrient-rich aquasoil or fertile soil, capped with 1.5–3 inches of medium-coarse sand to keep nutrients contained. Decorative gravel works only as a thin accent on top. Active aquasoils also lower KH and pH over time, which matters for shrimp and hard-water fish.
What substrate is best for Neocaridina shrimp?
Neocaridina do best on inert sand or gravel that does not alter KH or pH — stability matters more than enrichment for them. If you run an active aquasoil for plants, monitor hardness and expect gradual acidification. Walstad-style soil tanks are fine for shrimp too, but the soil must be capped with sand, never left exposed.
How many bags of aquasoil do I need?
Aquasoil is sold by volume — the common ADA bag is 9 liters — and it is much lighter than sand: roughly 0.85–0.95 kg per liter depending on brand, versus about 1.5–1.6 kg per liter for sand and gravel. So a planted bed needs fewer kilograms than the same volume of sand. Treat any single density number as a reference and weigh your own bag if precision matters.
What is the difference between US and UK gallons for tank size?
A US gallon is 3.785 liters; a UK (imperial) gallon is 4.546 liters — about 20 percent bigger. US tank sizes and rules of thumb (like 1 lb of substrate per gallon) assume US gallons. This calculator labels every gallon figure explicitly and lets you switch between the two, because mixing them up is one of the oldest mistakes in the hobby.
Fine sand or coarse sand for a fish tank?
Coarse, inert sand is the safer default: detritus sits on top for easy vacuuming, water circulates through the bed, and rooted plants can grip it. Fine sand compacts, clouds easily, and gives plant roots nothing to hold — a classic cause of chronically poor plant growth that is hard to diagnose later. Fine sand suits only specific inhabitants like sand-sifting gobies that need it.
Sources & rule confidence
Every rule baked into this calculator traces to a community source and carries a confidence grade. A = multiple independent community confirmations; B = consistent direction, edge cases remain; C = direction only, numbers are reference values. We show disagreements instead of hiding them.
- A5 · B True water volume = geometric volume minus substrate displacement minus air gap.
- A6 · A Rule of thumb: about 1 lb of substrate per gallon for a thin bed; planted tanks target 2-4 inches of depth instead.
- A7 · A Substrate is roughly 10% of tank volume; small tanks under 10 gal need a capped, thinner bed.
- A8 · B Sloped substrate volume uses mean depth (trapezoid); steep slopes are built with bagged lava rock, not extra sand.
- A10 · A Typical depth range is 1-4 inches; planted zones need 3-4 inches.
- A11 · A Inert sand deeper than 5 inches risks anaerobic zones unless countered by Malaysian trumpet snails, rooting plants, or coarse grain.
- A12 · A Layered beds: about 1 inch nutrient layer capped with 1.5-3 inches of medium-coarse sand; decorative gravel is a thin accent only.
- A21 · A Neocaridina shrimp need inert media that does not lower KH/pH; Walstad method requires soil plus a sand cap.
- D3 · A Sand-sifting gobies cannot be kept on bare bottom or pure crushed coral; they need a deep sand bed.
- D24 · A Choose substrate by four axes: inert vs active, plantability, color, and how easily detritus is vacuumed.
- D27 · A Standard US tank footprints: 20H = 24x12 in, 20L = 30x12 in.
- D31 · B Tank bottoms are not always flat rectangles — measure the actual base area.
- #5 · A Accident: anaerobic zone formed under deep inert sand.
- #6 · A Accident: fine sand does not support rooted plants.
- #15 · A Accident: substrate laid too shallow cannot be fixed later without a rescape.
- #17 · A Accident: wrong medium causes chronic poor plant growth that is hard to diagnose.
- #21 · A Accident: substrate laid too shallow — post-hoc fixes mean tearing the tank down.
| Medium | Density (kg/L) | Status | Sources |
|---|---|---|---|
| Medium-Coarse Sand | 1.55 | verified | CaribSea Super Naturals — 94 lb/ft³ official; Northern Filter Media pool sand — 100 lb/ft³ official |
| Fine Sand | 1.55 | verified | CaribSea Super Naturals — 94 lb/ft³ official; Northern Filter Media pool sand — 100 lb/ft³ official |
| Coarse Gravel | 1.59 | verified | CaribSea Jelly Beans (13-26 mm) — 99 lb/ft³ official; Northern Filter Media pool sand — 100 lb/ft³ official (adjacent band) |
| Pool Filter Sand | 1.6 | verified | Northern Filter Media — 100 lb/ft³ official; Groupe Bellemare pool filtration sand — 98 lb/ft³ official |
| Active Aquasoil (planted) | 0.9 | reference value | CaribSea Eco-Planted — 54 lb/ft³ = 0.865 kg/L official; Barr Report community weigh-in — ADA Amazonia ~0.95 kg/L |
Density verification detail and packaging notes: see src/data/substrates.js (audited 2026-10-16).
Disclaimer
All amounts are estimates for freshwater aquarium setup. Substrate depth and chemistry affect animal welfare — when a warning fires, resolve the condition before adding livestock. This calculator does not replace water testing or veterinary advice.