Water change
Replacing a portion of tank water with fresh dechlorinated water. The single most effective routine maintenance task. Typical schedule: 25-30% weekly for most freshwater community tanks.
Water changes serve three purposes: dilute accumulated nitrate, replenish trace minerals consumed by fish and plants, and dilute organic byproducts that test kits don't measure (DOCs).
The standard hobby practice — 25-30% weekly — is conservative and works for most lightly-stocked community tanks. Heavily stocked or planted tanks may need more frequent or larger changes; very lightly stocked tanks can sometimes do less.
Key rules: match temperature within ~2°F to avoid thermal shock; dechlorinate tap water (chlorine kills nitrifying bacteria and damages fish gills); never change more than 50% in one go without good reason — large changes can shock fish and disrupt biofilms.
Beginner-safe rule
Beginner-safe rule: 25% weekly with a dechlorinator (Seachem Prime or equivalent), water within 2°F of tank temperature. Skip more than 2 weeks and nitrate creeps up.
See also