What is the head height that you're pumping back up? In other words... how many feet does the pump have to pump water up? Also, how many water returns are you planning? 2? 4? As well, how many joints and elbows and unions are you planning? All of this affects the head pressure for a pump. A pump will not give you the same 100% GPH
rating if it has to pump water uphill, through joints, etc).
Figure this out, then you'll figure out what pump you should use. With a 75, again... estimating with two returns and a simple plumbing setup, I would go with a Mag 7. (I have a mag 9.5 on my 60 gal
and love it. It goes into 4 returns, meaning that there's 4 places in my tank where water is returned back into my main tank). HTH
I'll try to find a head pressure calculator online. This advice is just IME