Aquarium Water Change Calculator
Tested your nitrates and they're too high? Enter your numbers โ we'll tell you exactly how much water to change.
Calculate your water change
Assumes your replacement water has ~0 nitrates (typical for treated tap or RO water).
The dilution math
Water changes dilute pollutants linearly. The formula is simple:
Change % = (1 โ target รท current) ร 100
Example: nitrates at 40 ppm, target 20 ppm โ (1 โ 20/40) ร 100 = 50% water change. On a 20-gallon tank, that's 10 gallons.
Water change guidelines
| Situation | Recommendation |
|---|---|
| Routine maintenance | 20โ30% weekly |
| Nitrates 40โ80 ppm | 40โ50% change, retest in 2โ3 days |
| Nitrates over 80 ppm | Two 40% changes a few days apart โ never all at once |
| New tank (cycling) | Small 10โ15% changes only if ammonia spikes |
| Planted tank, light stock | 20% every 1โ2 weeks is often enough |
Water change best practices
- Never change more than 50% at once โ big swings in temperature and chemistry shock fish.
- Match the temperature of new water to the tank (within 2โ3ยฐF).
- Always dechlorinate tap water before it touches the tank.
- Vacuum the substrate while you siphon โ you're removing trapped waste, not just water.
- Clean filter media in old tank water, never tap water โ chlorine kills beneficial bacteria.
Frequently asked questions
How much water should I change weekly?
25% weekly is the standard for most tanks. Adjust based on nitrate tests: if nitrates stay under 20 ppm, your schedule is working.
What nitrate level is safe for fish?
Under 20 ppm is ideal for most freshwater fish; under 40 ppm is acceptable. Sensitive species and shrimp prefer under 10 ppm. Reef tanks should stay under 5 ppm.
Can I do too many water changes?
Frequency isn't the problem โ size is. Small frequent changes are very safe. The risk comes from huge single changes that swing parameters suddenly.