04-01-2011, 04:52 PM
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This can be problematic. Sometimes a particular fish is different, and male/female makes a difference too. If it is a male, it will, at 2.5 inches, probably consider the tank "his" and while other fish species may be tolerated (this can vary too), another angel, particularly another male, will be seen as a threat to "his" territory. Aggressive behaviour can be brutal, causing the death of the submissive fish. Other angels may tolerate the addition. On the side of probability, though, if it is a male it most likely will not allow others without a fight to the death.
As noted in our profile of Pterophyllum scalare, the common angelfish, they should always be acquired together as a group (5 or more) so that as they mature the natural pecking order can be established. Usually this works, but again some fish just will not tolerate others, especially males.
There was another thread on this just the other day, an aquarist had to remove an angel who was being attacked by the dominant male. And there were others similar not too long ago.
The only way to ascertain sex wiith angels is to observe the breeding tube; it is thicker in females, thinner in males. With the size of your fish, it may or may not be clearly visible.