Bulkheads -- Single or Double Threaded?

Colombian_Shark

King Catfish
Jan 23, 2006
82
0
0
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?
 
Yup, I plan to put a screen on top of the bulkhead, which will run down into the PVC through a hole drilled in the bottom of the aquarium.
 
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