Water-change frequency depends on tank type, bioload, and supplementation. Standard freshwater community: 25-30% weekly with a Python or 5-gallon bucket, dechlorinator dosed for the change volume, water within 2°F of tank temperature. Heavily-stocked freshwater (cichlids, goldfish): 30-50% weekly. Planted tanks: 30-50% weekly during fertilizer Estimative Index dosing - resets nutrient buildup. Saltwater FOWLR: 10-15% biweekly or monthly. Reef tank: 10-15% weekly OR 25% biweekly with reef-grade salt mix (Reef Crystals, Tropic Marin Pro Reef, Red Sea Coral Pro). Replenishes trace + dilutes pollutants. Quarantine tank: daily 25% changes with parameter-matched water during medication. Critical rules: always dechlorinate, always temp-match, never change more than 50% in one shot (osmotic shock), never skip a month then do 75% (parameter swing). If you fall behind, ramp up gradually - 30% then 30% next week, not 70% in one go.
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