How much substrate do I need?

Depth first: what is your tank for?

• Fish-only / easy-clean: 1-2 inches. Shallow beds keep waste visible and easy to siphon, which is the entire maintenance argument.
• Planted tanks: 2-3 inches. Enough depth for root systems to establish and anchor without pushing the water column too high.
• Heavy root feeders / layered scapes: 3-4 inches. For crypts, swords, and carpeted foregrounds where root space is a design constraint.
• Fine sand caveat: deep sand with no root structure or burrowing livestock can compact into anaerobic pockets, so keep pure sand beds on the shallower side.

Whatever the depth, slope it a little higher at the back — an inch or two of gradient reads as depth, gives foreground plants shallower rooting, and doesn't change your bag count much.

Gravel, sand, and aqua-soil are not the same weight

Substrate is sold by volume for soil (9-litre bags) and by weight for gravel and sand (commonly 5-pound bags), which makes apples-to-apples comparisons painful. The densities behind the numbers:

• Aquarium gravel — ≈ 1.6 kg/L. Inert, easy to vacuum, and the standard starter choice.
• Silica / pool-filter sand — ≈ 1.76 kg/L. Heaviest of the three; natural look, compacts more readily.
• Active aqua-soil — ≈ 0.88 kg/L. Roughly half the weight per litre, nutrient-rich, buffers and feeds roots, with a usable lifespan before it needs supplementing.

That's why a rule that's honest for gravel drastically overbuys soil: the same bed volume of aqua-soil weighs about half as much, and arrives in different-sized bags.

From depth to bags: the quick table

Figures below assume a flat 2-inch planted bed across standard footprints. Sloping beds or deeper layers add ~10-20%.

Notice the 29 gallon and 20 long share a footprint and therefore need the same substrate — proof that tank volume is the wrong input. Your substrate calculator asks for length, width, depth, and material so it can replicate this for your exact tank.

The familiar rules of thumb (and why they're rough)

• "1 lb per gallon ≈ 1 inch of bed." Handy for gravel in standard tanks, but it conflates capacity with footprint and ignores material density.
• "1.5-2 lb per gallon for a planted 2-3 inch bed." A workable planning range for gravel and sand; roughly half that for aqua-soil.

Both rules drift for long-and-low (55 gallon) or short-and-deep (breeder) footprints. If precision matters — and with substrate it cheaply can — do the volume calculation and skip the guesswork.

Substrate is also a volume thief

Every litre of substrate displaces a litre of water. A 2-inch bed in a typical 29 gallon tank displaces several gallons, and combined with hardscape this is a large part of why real water volume runs 10-15% below rated capacity. Whatever the calculator tells you for bag count, the same volume is water your fish will never swim in — measure your tank's net volume before dosing anything.

Work out your own bed in four steps

1. Choose a depth from the setup targets above.
2. Measure your tank's interior footprint in the same unit as the depth.
3. Compute the bed volume (length × width × depth), then convert to litres or weight using your material's density.
4. Divide by your bag size — 9 L for aqua-soil, 5 lb for gravel — and round up, adding ~10% for sloping and settling.

Heavier substrate also adds real pounds to your floor load, so feed the result into the weight calculator — a 75 gallon breeder with a 3-inch bed adds well over 100 pounds of gravel alone.

Rinse rules worth knowing

Gravel and sand carry fine dust that will cloud the tank for days — rinse them thoroughly first. Aqua-soil is the exception: most brands must not be rinsed, because washing destroys the granules and strips the nutrient charge. Add aqua-soil dry and fill slowly onto a plate or bag to keep it undisturbed.

Related reading: stocking and bioload · tank weight and floor load · net water volume.