What types of overflows do you have and what size are the drains? Also refresh me on your sump setup. Size and your bio media. I know the gars like the more calm water but with a 500g and that bioload, you should really be able to get a good turnover rate. and maybe even ditch one of those canisters. If your overflows can handle it, Id at least try to get something close to 3000gph. I have 2- 2" drians on mine and I run 7200 gph in pumps on the 750g with no problem. Its a bit loud when that second pump kicks on though. Also if your sump is big enough and you have enough biomaterial in it, look into ditching both canisters, and get 2 smaller pumps instead of one big one. That way if one craps out for some reason, there is always one still keeping the filter going.