How many GPH ???
 

I am new to the hobby and am building my first tank .. I'm at the stage where I need to purchase a pump for my refrigium which is located in the stand below the main tank ( about 3 ft/head). I have a 50g tank with a 30 gal refrigium. How many GPH should my pump be pushing ? how often should my tank cycle ?

Lots of people aim for roughly 5X turnover through a sump, but that depends on personal preference. Are you keeping a skimmer in the refugium? So if you go with a 5X turnover rate, with a flow rate of approximately 250GPH at 3' head, you could go with something like the Danner Mag-Drive 350 or something similar.

i'd say medium-low flow through the sump, and push your real waterflow using powerheads. My 120g tank is softie/lps/sps and it's pushing 4000gph through powerheads and 500gph through the sump, and I still feel it's not enough sometimes.

I think WHERE your flow is can be just as important as how much flow is in the tank. That's where I like powerheads because you can move them around and get them exactly where you need them. Everyone has their rock laid out different, so moving powerheads around for your own tank can provide flow exactly where you need it.