1 1/2" minimum. 1" too restrictive...you'll get 600 gph thru 1 1/2" comfortably.
.......attached to a mag 9.5
For a 75 gallon tank, a single 1" is fine. He doesn't need 600 gph through the sump.
what would be a good GPH flow through the sump? i am going to be running an algae turf scrubber as well but on its own smaller pump. also im not going to do any damage to my map pump if i am running it on a 1" return line am i?
Your return GPH should be equal to the GPH of your skimmer.
I have a 110 gallon with a Mag 9.5 and 1.5" return piping and it's plenty. That's why I say the same pump with 1" plumbing would be fine for your 75 gallon.
And no, you will not damage your Mag pump. In fact, your pump will draw less power with a lower flow rate.
Your return GPH should be equal to the GPH of your skimmer.
I have a 110 gallon with a Mag 9.5 and 1.5" return piping and it's plenty. That's why I say the same pump with 1" plumbing would be fine for your 75 gallon.
My Mag 9 on my 75g is plumbed with 1" pvc and 1" ID vinyl tubing. I have it tee'd off to my refugium and throttled back with a ball valve. I throttle it to about 300gph turnover between sump and display. Works like a charm for me.
Skimmers are rated in LPH, or SCFM.....how much air volume or intake the pump can draw and not water volume (notwithstanding manufacturers ratings/guidelines which can be taken with a grain of salt).
There's no correlation between the two. Whether you have a low or a high turnover thru your fuge is up to you.
what does tank size have to do with how much water flows thru a pipe?
This isnt a scalable guess based off metrics, its a calculation.
If you have a 10 gallon tank with a 1" overflow pipe, it will push thru the exact same volume as a 1000 gallon tank with a 1" overflow pipe if they're the same height.
If you are setting up an ATS, you should consider tee'ing off the supply line. Always try to eliminate add'l equipment where you can.
Good luck.
C