After having the 125-gallon tank I bought off ebay sitting empty in the corner for a couple of months, I've finally decided to put it up. The guy I bought it from sawed the bulkheads off (they were glued in), so I have to replace them. When I went online to buy them, I noticed there are both single and double-threaded varieations. Which one of these should I buy? What determines which one I need?