I recently got a 140 g tank from my buddy for 60.00. It came with the oak wood stand and vanity. My only problem is this is my first tank with 4 holes cut into the bottom. I have 1 big canister filter that has one hose on it than connects to 1 of the holes, then from there it connects to my pump and from my pump into the other hole. My question is what are the other 2 holes for? Am I missing something?