That is true, water changes only dilute the nitrates which still accumalate.
I try and do 100% water changes at least once a week but sometimes more than that. If I'm lazy I will do a partial but not long after, the tank receives a full 100%.
How often should be according to your water test.
I'm no expert, but I dont believe a 100% change is ever a good idea.
50% weekly is overkill too, unless you are overstocked, and dont have plants.
It may make a difference for different fish, but my polys are skittish for a week if I move a rock, so a 100% water change would be torture
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