ive got a 150g and i do a weekly 50% pwc... i could probably do more like a 25% if i wanted, but since my mbuna are so messy (and your oscars will be too), i just go ahead and do 50% to try and prevent any ammonia issues... im getting that filter soon, right now i have a fluval 405, 305, and an aqua-tech 30-60 running on mine, i just want to run 2 canisters instead of 3 filters lol... filtration is one thing you do not want to skimp on when it comes to bigger tanks. another reason im getting the fx5 is because im planning on upgrading this summer/fall when we move, im getting a 240g more than likely