Fish Tank Volume Calculator

Aquarium Setup Guide

Filtration Rule of Thumb

Filter capacity is one of the most undersold aspects of fish-keeping, and undersized filters are the most common cause of cloudy water, fish health problems, and tank crashes. The standard guidance: your filter should turn over the entire tank volume 4–6 times per hour (sometimes written as '4–6× turnover'). A 100-litre tank therefore needs a filter rated at 400–600 litres per hour minimum. For heavily stocked tanks (more fish than the basic stocking density suggests), or tanks with messy eaters like goldfish or large cichlids, aim for 8–10× turnover (800–1,000 L/hr for a 100L tank). For planted tanks with low stocking, 3–4× is sometimes adequate but the higher figure is rarely a mistake. The advertised filter rating is the maximum flow with the filter empty and pumping clean water — real-world flow drops significantly as media gets dirty, sometimes 30–50% lower after a few weeks. Buy a filter rated higher than your calculated minimum to account for this. External canister filters generally have higher real-world capacity than internal filters of the same nominal rating, because they have larger media volumes and aren't competing for space inside the tank. Multiple smaller filters can be more robust than one large filter (redundancy if one fails, distributed flow patterns), but a single oversized filter is simpler. Maintenance matters as much as size: a filter that hasn't been cleaned in 6 months is dramatically less effective than its rating suggests. Always run filters continuously — turning them off for hours kills the beneficial bacteria in the biological media that converts ammonia (toxic) to nitrite (also toxic) to nitrate (relatively safe). This calculator helps you calculate the right minimum flow rate from your tank volume.

Stocking Density

Stocking density rules for aquariums attempt to balance fish welfare against tank capacity, but the simple rules are very approximate and the right answer depends on the species. The classic rule of thumb is '1 cm of adult fish per litre of water' for small community fish — so a 100-litre tank could support 100 cm of fish (perhaps 10 fish averaging 10 cm). This rule works reasonably well for small, peaceful, slim-bodied community fish like tetras, danios, and small barbs, but breaks down badly for: large fish (which need more swimming space than just their body length), messy eaters and high-waste species (goldfish, large cichlids, plecos — which need much more water per cm of fish), aggressive or territorial species (which need more space to avoid stress), schooling fish (which need to be kept in groups of 6+ minimum), and unusually deep-bodied or active species. Better species-specific guidance from experienced aquarists: goldfish need 75–150 litres for the first fish and 40–75 litres for each additional (they grow much larger than people expect and are very high-waste); common plecos reach 30 cm+ and need 200-litre tanks minimum; angelfish need 100 litres for a pair; oscars need 300+ litres for a single fish. Most pet shops sell fish for tanks that are too small for their adult size — the cute juvenile becomes an unhappy or doomed adult. Research adult sizes and territorial requirements before buying. Under-stocking is much safer than over-stocking: under-stocked tanks have stable chemistry, healthier fish, and easier maintenance. Even mathematically 'within limits' tanks can have aggression problems when species clash. The 1 cm/litre rule is a starting point; species-specific reading from reputable sources (aquarium forums, fishkeeping books) is essential before committing to a stocking plan.

Medication Dosing

When medicating fish or treating water, the dose must be calculated from your actual water volume — not the nominal tank volume printed on the box, which often differs substantially. A 100-litre nominal tank typically holds 75–85 litres of actual water after accounting for substrate (gravel or sand 10–15 mm deep takes 5–10% of the tank volume), rocks and décor (typically another 5–10%), plants, equipment, and the empty space above the water line. So actual water volume is typically 10–20% less than the nominal tank capacity — sometimes more. Medicating to nominal volume therefore overdoses by 10–20%, which for some medications is dangerous (high zinc-based parasite treatments) or harmless (mild salt baths). For the most accurate medication dosing, calculate your actual water volume: measure the inside dimensions of the tank (not the outside, which adds the glass thickness), measure the water level from the substrate top to the water line, multiply length × width × water depth, and convert to litres. A standard 90 cm × 30 cm × 38 cm tank with water at 35 cm above the substrate holds 90 × 30 × 35 = 94,500 cm³ = 94.5 litres of actual water — versus its 100-litre nominal capacity. Then subtract for ornaments: estimate 5% for sparse decoration, 10% for moderate, 15–20% for heavily decorated tanks. A 100-litre nominal tank with moderate substrate and rocks therefore holds perhaps 80–85 litres of actual water; medicate to 80 litres to avoid overdose. For most medications, slight under-dosing is far safer than over-dosing; if in doubt, dose to the lower estimate and add more if needed. Always read the medication's specific instructions: some require multiple doses, removal of activated carbon (which removes the medication from the water), or specific parameters like temperature increase. This calculator gives accurate water volumes when you measure your tank correctly.

Preventive Veterinary Care

Regular preventive veterinary care reduces long-term costs and improves pet health outcomes. Annual vaccinations (dogs and cats) protect against serious diseases. Regular flea, tick, and worm treatments prevent infestations and parasite-borne diseases. Annual dental checks — dental disease affects over 80% of dogs and cats over 3 years old and causes chronic pain that is often undetected. Microchipping is compulsory for dogs in England and recommended for cats. Pet insurance taken out when pets