What should my GPH be?

And this will be a reef tank of course.

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You want to turn over the display volume 3-5 times per hour.

So a lot depends on your head height, what else you going to run off your return line, how many outputs on the return, and so on. Then you have to decide between internal pump or external pump......external will give you more options with more power for that size tank. Go with the biggest sump you can and the biggest skimmer you can......it makes things easier down the road.

10x was the standard formula 15 years ago wasn't it? Now, you will see many (RC threads) recommend to match your return with your skimmer pump output, with the theory that all your sump water will be conditioned before it enters the display again. Slower return as I understand from multiple RC threads and Mike Paletta allows for better nutrient absorption by charto.

In the end it probably doesn't matter that much, IMO. Successful reefs have been kept for many years with every conceivable variation of flow.

I would look at a pump that reduces power consumption if you choose to throttle it back on the return side.... Such as a Reeflo.

"In the end it probably doesn't matter that much, IMO. Successful reefs have been kept for many years with every conceivable variation of flow."

That's a true statement if I ever saw one lol. What it comes down to is there are a bunch of ways of making flow rates work in a reef tank if you know what your doing and have your system matched accordingly.

Ive tried the low turnover system before and I wasn't happy with it. I seem to get better results with high system turn over because I like to keep my tank severely minimally aquascaped with live rock and over stocked with fish so most of my filtration comes from a huge skimmer and enough water flow to feed it and my 4 chamber sump/ refugium. The other reason I like higher GPH return pump is it takes the place of two in tank power heads. I still have a little detritus that settles at the bottom of my refugium but my sand bed in my display tank stays clean and my skimmer cup stays full. Plenty of people disagree but It works for my tank.

I'll admit, as much of a research addict as I am, I either forgot or was unaware that people are now recommending 3-5 times the total water volume turnover rate. One advantage to buying a larger pump is the ability to have it as a multi purpose pump. Like having it feed multiple tanks, reactors, ect.

I've tried all the above and I recently bought a jebao DC9000 for my return pump on a 75g display if that tells you anything.
With DC pumps everywhere right now you're best bet would be to get a pump that you know has a happy medium so you don't have to upgrade or downgrade. Plus they use less watts
This pump maxes out at 3140gph at 0 head. Since it has 6 speed settings and you can go from 900gph to 3140gph. So you'd be able to go anywhere between 3x turnover to around 10x turnover depending on head hight.
www.amazon.com/gp/aw/d/B00INTDJDA/ref=pd_aw_sbs_2?pi=SL500_SS115&simLd=1
Should note though that reviews are mixed on these. Some people love them and some don't.

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Manifold, something like this:

those are plugs before reactors were installed.