700 gallons and 500 fish? yikes!
how much air do you have going in?
how big are these fish? and yeah, you need gobs of filtration..
what kind of turnover in the tank? (how often does the volume of tankwater turnover?
with that many fish, i'd have pure o2 injection..
i have a 700 gallon tank (or so) and would be pushing it to capacity with 200 adult fish..
oh, and do you have some kind of backup power?
I'm with Keith on this one. Not sure how you been getting by with it but you are now seeing what's gonna happen. You have waaaaaaaay to many fish in there at 4 inches.
Hate to say it but a good rule of thumb is to use the 1 inch per gallon rule. Anotherwards if you have a 10 gallon tank you can have 10 inches of fish. No it's not a perfect system but it does help keep you close.
So in your case 700 gallons, take that number and divide by 4 cause you said the fish are 4 inches, and you end up with 175. So there you go. Hate to say it but Keith is right! Well I don't hate to say it cause I don't even know Keith

but anyway he is right on. You should not have no more than 200 fish in that tank!
RC