Tank volume & filter turnover calculator

Why accurate volume matters for your planted tank

Tank manufacturers size their products by nominal capacity — the volume a tank would hold if filled to the brim with no substrate, no hardscape, and no fill-level shortfall. In practice, a tank marketed as "90 litres" often contains 15–25% less actual water than that figure suggests.

Glass thickness reduces the internal footprint. A typical 6mm pane on each wall of a 90 × 45 × 45 cm tank removes over 3 litres before you add anything. Five centimetres of substrate across the full footprint removes another 12–16 litres depending on substrate type. Fill the tank to 90% rather than the brim and you lose another 8 litres. The final net water volume can easily be 25–30 litres below the nominal figure.

This matters because CO₂ dosing, liquid fertiliser doses, and medication are all per-litre calculations. Dosing against an inflated nominal volume leads systematically to understrength treatment — plants get less fertiliser than calculated, CO₂ targets are reached at a lower bubble rate than expected, and any medication prescribed "per litre" is diluted by the inaccuracy.

Why substrate type changes the displacement calculation

Not all substrate displaces the same volume of water per centimetre of depth. The key variable is void fraction — the proportion of the substrate layer that is actually empty space between grains, filled with water rather than solid material. Only the solid fraction displaces water from the tank.

Fine sand packs tightly with around 35% void space, meaning roughly 65% of its apparent volume is solid and displaces water. Aquasoil granules are larger and more irregular, leaving around 45% void space — so the same 5cm depth of aquasoil displaces noticeably less water than 5cm of sand. Lava rock substrate is extremely porous at up to 60% void, making its displacement much lower than other types at the same depth.

On a 90 × 45cm footprint with 5cm of substrate: fine sand displaces approximately 13.2 L, medium gravel 12.2 L, and aquasoil 11.1 L. The difference of 2 litres matters when calculating CO₂ dosing and fertiliser concentrations.

Hardscape and the displacement effect

Rocks displace a meaningful volume of water. The exact amount depends on the density of the stone — denser rocks displace less water per kilogram than porous ones. Seiryu stone, one of the most popular aquascaping stones, has a density of roughly 2.7 g/cm³, meaning each kilogram displaces about 0.37 litres. A 10 kg Seiryu scape displaces roughly 3.7 litres of water.

Dragon stone (Ohko) is significantly more porous, with a density around 1.5–1.8 g/cm³ depending on the piece. A 10 kg dragon stone arrangement displaces 5.5–6.5 litres. For heavily scaped tanks, particularly iwagumi styles with substantial stone masses, hardscape displacement can account for 5–10 litres of reduced water volume.

The calculator includes density data for common stone and wood types used in aquascaping, grouped by material: limestone and sandstone (Seiryu, Pagoda, Frodo), volcanic and porous stone (Dragon stone, Elephant skin, Lava rock), dense igneous and metamorphic stone (Slate, Yamaya, River pebbles), and driftwood. You can add as many hardscape items as you have — rocks and wood are tracked separately. For unusual materials, enter the displacement in litres directly using the "Other" option.

True system volume vs net water volume

Your external filter holds water too — water that participates in the system and is replaced during water changes. The Eheim Classic 2217 holds approximately 3 litres of water in its canister and hoses. The Fluval 307 holds around 2.5 litres. For a 60-litre net tank volume, a 3-litre filter represents a 5% addition to the system — significant for CO₂ concentration calculations.

For water change percentage calculations, the net water volume in the tank is what you drain and refill, so use the net water volume. For CO₂ dosing and fertiliser target concentrations, the true system volume (tank + filter) gives a more accurate target, because CO₂ and dissolved nutrients distribute throughout the entire water volume including the filter circuit.

Once you know your tank volume, use the substrate calculator to find out how many bags of aquasoil or gravel your footprint needs at your target depth. Then use the CO₂ cylinder runtime estimator to find out how long your cylinder will last based on your tank's CO₂ demand.

Understanding filter turnover rate

Filter turnover rate is how many times per hour your filter circulates the total system volume. It is calculated by dividing the filter's effective flow rate (in litres per hour) by the true system volume — the net water volume in the tank, minus hardscape displacement, plus any water held in the filter canister and hoses.

Using the system volume rather than the nominal tank size gives a more accurate turnover figure, because all that water — including the volume inside the filter — participates in circulation. A 3-litre canister filter on a 60-litre net tank represents a 5% addition to the system that affects the real turnover rate.

The manufacturer rating vs actual flow

The difference between rated and real flow matters for turnover calculations. An Eheim Classic 2217 is rated at 1000 L/h — at 70% real-world efficiency that delivers approximately 700 L/h. On a 80-litre system that is 8.75× per hour rather than 12.5×. If you have measured your actual flow rate (by timing how long the filter takes to fill a bucket of known volume), select "Exact / measured" and enter that figure for the most accurate result.