Per Change
7.3 gal
Frequency
Every 7 days
Monthly
32 gal
| Tank Type | Change % | Frequency |
|---|---|---|
| FW (Lightly Stocked) | 15% | 14 days |
| FW (Moderately Stocked) | 25% | 7 days |
| FW (Heavily Stocked) | 30% | 5 days |
| Planted Tank | 30% | 7 days |
| SW (FOWLR) | 15% | 14 days |
| SW (Reef) | 10% | 7 days |
Inputs
Result
A 29-gallon moderately stocked freshwater tank needs 7.3 gallons (25%) changed weekly. At about 4.3 changes per month, the annual dechlorinator cost is under $5.
Inputs
Result
A 75-gallon reef tank does 10% weekly changes (7.5 gallons). With RO/DI water and reef-grade salt mix at $0.50/gal, annual water change supply costs are about $211.
Most freshwater aquariums need a 25% water change weekly. Lightly stocked tanks can do 15% biweekly, while heavily stocked tanks need 30% every 5 days. Saltwater reef tanks do best with 10% weekly to maintain trace element stability.
| Tank Type | Change % | Frequency | For 29 gal Tank |
|---|---|---|---|
| FW Light Stock | 15% | Biweekly | 4.4 gal |
| FW Medium Stock | 25% | Weekly | 7.3 gal |
| FW Heavy Stock | 30% | Every 5 days | 8.7 gal |
| Reef Tank | 10% | Weekly | 2.9 gal |
Weekly water changes are the gold standard for most aquariums. The exact frequency depends on stocking level, filtration, and whether you have live plants. Test nitrate levels: if they exceed 20 ppm before your next scheduled change, increase frequency.
Yes, tap water must be dechlorinated before adding to your aquarium. Chlorine and chloramine kill beneficial bacteria and harm fish. Use a water conditioner like Seachem Prime at 1 ml per 10 gallons. RO/DI water does not need dechlorination.
| Water Source | Dechlorinator? | Remineralize? | Cost/Year (29 gal) |
|---|---|---|---|
| Tap Water | Yes | No | ~$5 |
| RO/DI Water | No | Yes (FW) | ~$15 |
| RO/DI + Salt | No | N/A | ~$95 |
Yes, changing more than 35–40% at once can stress fish by rapidly shifting pH, temperature, and mineral content. For emergency situations like ammonia spikes, do two 25% changes 2–3 days apart rather than one 50% change.
Freshwater tap water changes cost about $5–10/year in dechlorinator. Saltwater tanks are more expensive at $80–200/year because of salt mix costs. RO/DI systems add $10–20/year in filter replacements.
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Last Updated: Mar 9, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.