If the tank is not set up, you really should get it drilled. Its not hard to do. If you do use a siphon overflow, use a Lifereef. They are the best, most reliable out there. Since it is a 6' tank, I would want either two overflows, one one each end, or one large one in the middle to get good surface skimming. for flow, with dual overflows, or 1 large one, you would have 2 U Tubes. I like to see a minimum of 200 GPH through each U Tube, so no less than 400 GPH. 500 - 600 GPH would be good.