I have had my tank about 3 years, so all cycled and perameters are always consistently good I just wondered how often I should do it? ATM I do 50% twice a week but I have heard that some people do it at least three times a week in a predator tank, I have in there 2 small oscars, baby red tail cat, baby tsn cat, giraffe cat and gibbiceps pleco, it's 350l (85usg) with a 1000 LPG canister filter. Any info appreciated