How deep should the bed be?
Fish-only tanks with inert gravel or sand do fine on 3–5 cm—enough to look natural and anchor décor without trapping excessive waste. Planted tanks want 5–8 cm so roots can spread, often sloped shallower at the front and deeper at the back for depth perception. Undergravel-filter setups (now rare) need 6–8 cm. Deeper than ~8 cm of fine sand risks anaerobic pockets; if you want tall banks, build the core from coarse material or lava rock and cap it.
Litres to kilograms by material
Materials pack differently: dry quartz sand ≈ 1.5–1.7 kg per litre, standard aquarium gravel ≈ 1.4–1.6 kg/L, crushed coral ≈ 1.2 kg/L, and lightweight aquasoils ≈ 0.8–1.0 kg/L. So the same 10-litre bed weighs 15 kg in sand but only 9 kg in aquasoil. Bags list either litres or kilograms—convert to the same unit before comparing prices, because "cheap" heavy gravel and "expensive" light aquasoil are closer per litre than they look per kilo.
Slopes, hardscape and buying margin
For a sloped scape, use the average depth in the formula: front 4 cm rising to 8 cm at the back averages 6 cm. Large rocks and wood sitting directly on the tank base displace substrate—heavily hardscaped layouts often need 10–20% less than the flat calculation. Buy one bag beyond your calculation anyway: topping up later rarely matches colour and grade exactly, and leftovers are useful for rescaping and quarantine tanks.
Washing and weight matter too
Rinse inert gravel and sand until the water runs clear—skipping this clouds a new tank for days. Aquasoil is the exception: never rinse it, just pour and fill gently over a plate. Remember the substrate adds real weight: a 20-litre bed is ~30 kg of extra load on the stand, and it also displaces water, reducing your true water volume—which matters when you dose treatments.