On my 125 I have one hole for the drain, it is 2 1/2" for one 1 1/2" bulkhead, my overflow is a coast to coast so it can handle way more than most pumps, you really don't need high flow through a sump, actually you probably want a slower flow. I'll be drilling a hole for a 1" BH in the side of my 58g sump when I get it & use an external pump, IMO less heat, better flow options, less to keep clean also. On a 150G tank, I would set up for two 1" drains or 1 1/4" even better, the pipe I.D. is large enough it should not have any plug up problems & small fish fit through them rather easily, oh it can happen. I had my regal (Hippo) stuck in there all day and somehow lived, in my 100g tank with a single 1" drain. If you have small overflows use the stockman design, easier to make & adjust, water should be almost completely silent if done right in the standpipe.