I change about 40% on both the 29G fancy goldfish tank and the 150G tropical community, whenever I get round to it. Somewhere between weekly, and fortnightly, occasionally they will go three weeks, depending on how busy I am.
Both are fairly heavily stocked (hence me saying yes to the fish influencing it), and as Tolak says, fresh water really does make a big difference to ensuring healthy fish.
Both are fairly heavily stocked (hence me saying yes to the fish influencing it), and as Tolak says, fresh water really does make a big difference to ensuring healthy fish.