You want aprox 5-10 times of your total water volume going thru your sump. So with a 75g, you are looking at anywhere from 385gph to 750gph.
Flow inside the tank is another story. You can go up to 40 times or more of your tank volume for SPS
corals. You can easily use 2 K
-4's in your tank.
I have 4 K
-4's and 1 K
-2 in my 125 for a total of aprox 5400 gph
, not counting my returm pump.