Aquarium Calculators & Tools
Whether you're setting up your first tank or dialing in a mature reef system, these free calculators take the guesswork out of the most common aquarium math. No signup, no ads, no nonsense — just the numbers you need.
How to Use These Tools
Each calculator below is interactive — just enter your tank's measurements or parameters and the result updates instantly. Start with the Tank Volume Calculator if you're setting up a new tank, since most other calculations depend on knowing your exact volume. For reef hobbyists, the Salinity Converter and CO2 Calculator are particularly useful for dialing in water parameters. All tools work on mobile — bookmark this page for quick access at your local fish store.
Enter your tank's pH and KH (carbonate hardness) to estimate dissolved CO2. Ideal for planted tank hobbyists using pressurized CO2 systems.
Estimated CO2
12 ppm
Too Low
Plants will grow slowly. Consider adding CO2.
Suggested starting bubble rate: ~2 bps for this tank size
Common Aquarium Questions
Answers to the questions hobbyists ask most — at the fish store, in the forums, and everywhere in between.
How many gallons is my aquarium?
For a rectangular tank, multiply length × width × height (all in inches) and divide by 231. A standard 48"×12"×16" aquarium equals about 39.8 gallons. Use our Tank Volume Calculator above for instant results including cylindrical and bow-front tanks.
How often should I do water changes?
Most freshwater tanks benefit from a 25–30% water change weekly. Heavily stocked tanks or reef systems may need more frequent changes. Lightly stocked, well-planted tanks can often go every two weeks. Consistency matters more than exact percentage — the same schedule every week is better than irregular large changes.
What wattage heater do I need for my aquarium?
A common starting point is 5 watts per gallon. However, the temperature difference between your room and target water temp matters significantly. A tank in a cold room needs more wattage than one in a warm home. Our Heater Size Calculator accounts for this delta automatically.
What is the ideal salinity for a reef tank?
Most reef aquariums are kept at a specific gravity of 1.025–1.026, which equals approximately 33–35 ppt salinity. Fish-only saltwater tanks can be kept slightly lower at 1.020–1.025. Always use a quality refractometer rather than a swing-arm hydrometer for accurate readings.
How many fish can I put in my tank?
The classic "1 inch of fish per gallon" rule is a rough starting point for small community freshwater fish, but it breaks down quickly for larger fish, cichlids, and saltwater species. Bioload (waste production), swimming space, and aggression matter as much as size. Our Stocking Density Guide gives type-specific guidance.
What CO2 level is safe for a planted tank?
The ideal dissolved CO2 range for planted tanks is 15–30 ppm. Below 15 ppm, plant growth slows noticeably. Above 35–40 ppm, fish may show signs of stress (gasping at the surface). CO2 levels are estimated from pH and KH using the relationship CO2 = 3 × KH × 10^(7–pH).
Do I need to dechlorinate tap water for water changes?
Yes — always. Municipal tap water contains chlorine or chloramine, both of which are toxic to fish and beneficial bacteria. Use a quality dechlorinator (sodium thiosulfate treats chlorine; a product containing Prime or similar treats chloramine). Add it to the new water before it goes into the tank.
Where can I find a local fish store that tests water for free?
Many independent local fish stores offer free water testing — it's one of the biggest advantages of shopping local over online retailers. Use our directory to find stores near you and look for the "Water Testing" service tag on store profiles.
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