Aquarium Calculators and Fish Tank Tools
Free tools that start from your real water volume, not the number on the box. Measure your tank once and every calculator remembers it.
Start: measure your tankTank setup
Size the tank first, then match the filter, heater and other equipment to the water you actually have.
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Aquarium Size Calculator
Tank volume in litres and gallons for 6 shapes, the real water volume after substrate and decor, and the full weight on your stand.
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Aquarium Filter Flow Calculator
Turnover per hour from your filter’s GPH or LPH, checked against your fish, reef or pond, plus the flow you need and multi-filter totals.
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Aquarium Heater Calculator
Heater wattage from tank size, room temperature and target temperature, with standard sizes and a two-heater split.
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Aquarium Substrate Calculator
How much gravel, sand or aquasoil to buy for your depth, including sloped scapes.
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Aquarium UV Sterilizer Calculator
Check if UV suits your problem, find the right flow rate and test your unit against its manufacturer rating.
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Aquarium Light & Photoperiod Calculator
A lighting schedule matched to your plants, light strength and algae situation.
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Fish Tank Size Guide
Pick your fish and get the minimum and recommended tank size, length and height they need.
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Water quality
Plan water changes, keep nitrate down and dose treatments for your real water volume.
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Aquarium Water Change Calculator
Litres or gallons to remove and replace, the new nitrate, ammonia or GH after a change, multi-tank totals and a dated schedule.
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Aquarium Salt Calculator
Salt to add from volume, current and target concentration: freshwater salt, marine mixes, water changes and g/L, ppt or SG.
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Aquarium Evaporation Calculator
Daily water loss, top-off volume and salinity drift for open and lidded tanks.
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GH & KH Calculator
Convert dGH, dKH and ppm, and mix RO and tap water to a target hardness.
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Aquarium Dosing Calculator
Scale any product label dose to your real water volume.
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Aquarium Cycling Tracker
Log ammonia, nitrite and nitrate and see which stage of the cycle you are in.
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Fish and stocking
Check how many fish fit, which species get along and what temperature suits them all.
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Fish Stocking Calculator
Stocking level by species, with warnings for school size, tank size and water needs.
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Fish Compatibility Checker
Check temperament, size and water overlap for every pair of fish you plan to keep.
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Aquarium Temperature Checker
Check your water temperature against your fish, shrimp or reef range, with steps if it is too hot or cold.
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Problems
Work out what is going wrong when the water turns cloudy, fish gasp or algae takes over.
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Aquarium Problem Checker
Cloudy water, gasping fish or white spots: likely causes and safe next steps.
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Aquarium Oxygen Checker
Rate your tank’s low-oxygen risk from temperature, stocking and surface movement.
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Aquarium Algae Identifier
Identify the algae in your tank and fix the cause, not just the symptom.
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How we build our aquarium calculators
- Real water volume first. Glass, substrate and decor take up space, so every tool starts from the water you actually have, not the box size.
- Named sources. Water quality targets come from the Merck Veterinary Manual and OATA, unit conversions from NIST, and heater figures are checked against manufacturer charts.
- Formulas on the page. Each tool shows how it calculates, with a worked example you can check by hand.
- Private. Calculations run in your browser. Your tank is saved only on this device.
Frequently asked questions
Which aquarium calculator should I use first?
Start with the Aquarium Size Calculator. It works out your real water volume and saves it, so the water change, filter, heater, UV, salt, dosing and other calculators start from the right number.
Why use real water volume instead of the tank size?
Glass, the gap below the rim, substrate and decor take up space. A "60 litre" tank often holds 45–50 litres of water, and doses, water changes and heater sizes should be based on that.
Do the calculators work in gallons?
Yes. Switch between metric and US units at any time. Results show litres, US gallons and UK gallons.
How do I check which fish I can keep together?
Use the Fish Compatibility Checker to compare every pair, then the Fish Stocking Calculator to see how many fit in your tank. The Fish Tank Size Guide lists the minimum tank for each species.
Something is wrong with my tank. Where do I start?
Open the Aquarium Problem Checker, pick what you see and enter any test results. For gasping fish, also run the Aquarium Oxygen Checker.
Are these aquarium tools free?
Yes. They run in your browser, need no account, and only save your tank on your own device.
Looking for other pet tools? See all animal calculators, or identify a fish from a photo with the fish identifier.
