Hi moga
If you're saying you have two pumps, one feeding the refugium and the other returing water to display tank - how is it all hooked up ?
Trying to match flowrates of two pumps, even if you use balancing valves, is tricky if not near impossible. Eventually you'll start to see the offset - or error between the two - and it could be a wet mess.
Can you please describe details of your set up, and maybe we can all look at some simple options.