Just to add, you do not "HAVE" to drill holes in your tank, it is better, but you don't have to, you can just run a outlet tube in the tank and then plumb in the inlets over the top of the tank.
Typically one hole in the middle of the back of the tank is where the closed loop hole is, look up bulkhead fitting, you will put this in the hole, you can buy a strainer to put on the tank side of your closed loop so it doesn't suck any fish in the loop. Then the water goes to the pump then typically to at least two inlets back into the tank, you can drill holes for them or just plumb them over the top of the tank. Another thing to look at is an oceansmotions unit this alternates between 4 inputs, and creates random water movement in the tank.