Picking a pair is very difficult unless they are going to spawn at which point the pair will take over 1 end of the tank and not allow any other fish into the spawning area. I have even seen them try to take over 3/4 of the tank. At this time their breeding tubes will be exposed on the bottom of their bellies. The females will be much bigger than the males. This is the only sure way to sex angelfish. Any other way is just guessing. Keep in mind it is not uncommon for 2 females to pair up either, that is why you must look at their breeding tubes.