I change something like 30-40% of water a day on my tank, its a lot of water to change but i've read a few places fresh clean water with lots of water changes should keep the fish healthy and help them grow faster. Fish have been in there for a few months and had no ill affect so far on any of the fish.
I might change it down a little if the electricity bill is to high heating the tank but i never notice them lit up so shouldnt be too much of an increase.
As for flow rate i just used a 4 galon bucket, timed it filling it to a certain amount to work out the flow.
I have overflows in the tank that go to the sump and then an overflow in the sump which go's directly into the drain.
I might change it down a little if the electricity bill is to high heating the tank but i never notice them lit up so shouldnt be too much of an increase.
As for flow rate i just used a 4 galon bucket, timed it filling it to a certain amount to work out the flow.
I have overflows in the tank that go to the sump and then an overflow in the sump which go's directly into the drain.