Hi
It's hard for anyone to advise without seeing your setup and witnessing how the flow is arranged and affected by decor etc. But as has been said, a much higher turnover is possible so long as you're managing the flow to avoid this happening with your angels. I have my additional flow from a newave pump on for the period the CO2 is on, but its below half height and it's aimed from about the middle of the left glass to the middle of the rear glass so when it hits the front it's much more manageable for the fish. So what I've achieved, is to offer a flow the fish wish to avoid which is in the dark rear of the tank, hence they hang in the front where its comfortable to swim and I can see them
once the lights go out this pump is switched off anyway, so they can go where they like. But, my fish seem to occasionally like this current and will disappear to the back, swim up the current then back out to the front. Cool!
On my 3ft tank which is 18" high and 14" wide I run a filter that's way overpowered, a fluval 405. When I first set it up and turned it on, it flattened the vallis in the far right corner and that was coming from a spraybar at the front left. I wanted the spraybar here to avoid too much disturbance of the vallis so I removed it and drilled another 2 rows of holes. Its now quite a slow flow through the whole tank, and to be honest I will aim to increase it slightly. One day.
So take a look at your output or outputs and have a re think, and with a bit of adaption you'll be wondering where all that flow has gone, and you'll probably gain a more uniform distribution of flow throughout the tank.
Cheers
Gavin