How to size a pump to your tank

  • We are currently upgrading MFK. thanks! -neo
hey guys i'm kinda new to this but can someone tell me i have a 30 gal saltwater tank and i'm about to make a wet/dry system for it from a 20 gal tank what size pump for the return should i get or how much gph pump
 
I recently upgraded my Oscar tank from my 55 to a 125 long. He is around 6" and so is my pleco. I added a jack Dempsey, a green terror, and a flower horn, all around 3. The Oscar is def the boss. And aslong as the others stay away the jd's cave, It's actually a peaceful tank. My question is when these guys grow up will my fluval fx5 be good enough filtration?
 
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