Free water change calculator. Enter tank size + nitrate reading + target nitrate, get the water change percentage and frequency to maintain target. Works for freshwater + saltwater + reef.
This calculator uses industry-standard formulas validated against published reef + freshwater husbandry references. Enter your actual numbers above for results. The output is meant as a starting recommendation - adjust based on your tank's specific bio-load, equipment, and observed parameter trends.
Save the result: screenshot or copy the recommendation. Re-run the calc weekly while you dial in your tank, then move to monthly checks once stable.
Browse the full Fast Aquatics calculator library. Other tools: tank stocking, water change, 2-part dosing, PAR estimator, electricity cost.