im running a 2640 on my tank internally at home , love these pumps. i run 3/4" lines from my pump and the smallest barb fitting they give you fits inside the pipe snuggly.
i run mine on speed 4 and have my return t'd off for a manifold if i ever plan to add something, just up the speed a bit.
at the store im running a few different dc pumps and skimmers, we just hooked up two externally externally and on a closed loop system , sched 40 adapters fit the threads perfectly and they do not leak one bit so far.we have both of them on a controller to run 30 sec back and forth.
the dc pumps do have a small humm to them but nothing compared to some of the heavy ac pumps of equal size.
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