Almost every aquarium decision starts from volume — how much you dose, how big a filter you need, how much water you change, and how much you can stock. The number on the box is the tank's nominal size, not the water in it. This calculator works out the gross volume from your measurements, then the usable volume after the rim gap and the water your substrate and decor displace.
A tank is never filled to the rim, and substrate, rocks, wood and decor all displace water. Between the two, the actual water volume in a furnished tank is typically ten to fifteen per cent below the gross internal volume.
That matters most when you are dosing. Medication, fertiliser and dechlorinator are dosed per litre or per gallon, and dosing a 200 litre tank that actually holds 175 litres of water means every dose is over-strength. For medication in particular that is worth getting right.
The calculator takes twelve per cent as the middle of the usual range. If your tank is heavily aquascaped with large rock, assume more; if it is bare-bottomed, assume less.
Turnover is how many times the tank's water passes through the filter each hour: filter flow rate divided by tank volume. Four times an hour is a comfortable minimum for a normally stocked community tank, six to eight suits a well-stocked one, and ten or more is what messy fish like goldfish and large cichlids need.
Two cautions. Manufacturers usually publish the pump's free-flow rate, not the flow through loaded media — real turnover is lower, and falls further as the media clogs. And flow is not the same as filtration: what actually keeps ammonia at zero is the volume of biological media in contact with the water, which is why a canister outperforms a hang-on-back of the same nominal flow.
High turnover is not automatically better. Bettas, fancy goldfish, gouramis and other long-finned or slow fish struggle in strong current and will spend the day pinned in a corner. Aim the outflow along the glass or diffuse it if your fish are being pushed around.
A weekly change of twenty to thirty per cent of the water volume suits most community tanks. Heavily stocked tanks want more; lightly stocked planted tanks can often do less.
Match the temperature of the new water to the tank, and treat it with a dechlorinator before it goes in, or at the same time. Chlorine and chloramine kill the nitrifying bacteria in your filter as well as irritating fish.
Never rinse biological filter media in tap water. Rinse it in water you have just siphoned out of the tank. This is the most common way people accidentally restart a cycle in an established tank.
The old inch-per-gallon rule is unreliable, because a fish's waste output does not scale with its length and a schooling fish needs swimming distance rather than volume.
Footprint often matters more than volume. Active swimmers and schooling species need length; bottom dwellers need floor area; tall tanks give you height that most freshwater fish will not use.
Research the adult size and the group size of any species before buying, not after. Most stocking disasters are a fish sold small that grows, or a schooling species bought in twos.
Multiply length by width by height in centimetres and divide by 1,000 for litres. Divide litres by 3.785 for US gallons. Then take ten to fifteen per cent off for the rim gap and the water displaced by substrate, rock and decor — that is your real water volume for dosing.
Four times the tank volume per hour is a comfortable minimum for a normally stocked community tank, six to eight for a well-stocked one, and ten or more for goldfish and large cichlids. Remember that published flow rates are free-flow figures and real turnover through loaded media is lower.
Twenty to thirty per cent of the actual water volume suits most community tanks weekly. Heavily stocked tanks need more. Match the temperature and treat the new water with dechlorinator — chlorine and chloramine harm fish and kill filter bacteria.
Because dosing is per litre. Medication, fertiliser and dechlorinator dosed to the tank's nominal size in a tank that actually holds ten to fifteen per cent less water are all over-strength, and with medication that margin matters.
No. Waste output does not scale with length, and schooling fish need swimming distance rather than volume. Footprint usually matters more than volume — research each species' adult size and group needs before buying.