Aquarium Substrate Depth Calculator

Why substrate depth matters:

Substrate does far more than decorate the tank. It anchors live plants, houses a large share of the beneficial bacteria driving the nitrogen cycle, and in the case of soils it shifts water chemistry as well.
Too shallow and plant roots have nowhere to go. Too deep and the bottom of the bed runs out of oxygen, which lets sulfate-reducing bacteria take over and produce hydrogen sulfide, the rotten-egg gas. In fine sand that starts becoming a risk past about 3 inches; coarse gravel keeps enough flow through it to go deeper.

Recommended depths by setup:

Volume formula:

Volume (liters) = Length (cm) × Width (cm) × Depth (cm) ÷ 1,000

Volume (gallons) = Length (in) × Width (in) × Depth (in) ÷ 231

Weight estimation by substrate type:

• Gravel (pea-sized): ~1.5 kg per liter (~12.5 lbs per gallon), sold in 20 lb bags
• Sand (pool filter): ~1.6 kg per liter (~13.3 lbs per gallon), sold in 20 lb bags
• Aqua soil (ADA, Fluval): ~0.9 kg per liter (~7.5 lbs per gallon), sold in 9-litre bags
• Eco-Complete: ~1.2 kg per liter (~10 lbs per gallon), sold in 20 lb bags

Note which unit each one is sold in, because it is not consistent. Gravel, sand and Eco-Complete go by weight, aqua soil by volume, and a 9-litre bag of aqua soil goes about half again as far as a 20 lb bag of gravel, which works out to 6 litres.

Worked example:

A 36 × 18 inch (91.4 × 45.7 cm) tank with 3 inches (7.6 cm) of planted substrate:

• Volume = 36 × 18 × 3 ÷ 231 = 8.4 gallons, which is 31.9 liters
• If using aqua soil at 0.9 kg/L: 31.9 × 0.9 = 28.7 kg (63 lbs)
• Aqua soil comes in 9-litre bags, so buy 4 (the arithmetic says 3.5, and bags do not come in halves)

Watch the unit conversion on that first line. A 36-inch tank is 91.4 cm, not 90, and 3 inches is 7.62 cm rather than 7.6. Rounding both down the way product listings usually do costs you about 3% of the volume, which on this tank is a litre of soil you did not buy.

Sloped substrate tip:

For a natural aquascape look, run 1 inch at the front and 4 inches at the back, then calculate on the 2.5-inch average.
The slope does two jobs. It exaggerates the sense of depth in a shallow tank, and it keeps the front glass clear so you can actually see whether the bed is staying clean.

Capping layers:

If using a nutrient-rich base layer (like peat or mineralized soil), cap it with at least 1 inch of gravel or sand to prevent the nutrients from leaching into the water column and causing algae blooms.