What size pump for a 40 gal>>>>>

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Hello all,

I am currently working on a 40 gal. breeder frag set up.
I am trying to copy Mr. Ugly's setup (if you are familiar).

I have built a refugium out of a 20 gal long.

What size pump would you reccomend? How many gal. per hour?

The sump sits about two feet below the main.

Thanks,
Steve