My male convict just killed the 3rd female he has been with, and that's the last straw. What can I do with him? I would rather not kill him...he is really fun and he gets excited when I come in the room, but I want to use the 10g to breed something.
You gotta go with a bigger tank, 10g is tricky maybe trade him in and get a very small new pair. Chances are though you will eventually end up with the same problem.
I would say, try it in a bigger tank, there just might not be enough room in that tank for him and another fish. If he is as good as you say he is, it might be worth investing the extra few $ to keep him in a bigger tank, especially if you are attatched to him.
There are quite a few other fish you could try and breed in a 10g.
They wont do very fine in a 10g, atleast a 20 is preferable, if you add a female into a males territory even in a larger tank then u'll have to sacrifice it. Get the female add it into the tank 2-3days(or till it gets comfortable and homely) before adding the male, add some decor for hiding places which is required to save it from the male always.
You're keeping them in a 10g, thats the problem. I suggest at least a 20g long or a 29g. IMO a ten gallon is not useful for breeding anything, not even guppies. Not unless you have alarge growout tank. Take the fish back to the store, a 10g is no good home for a fish that can reach 6". That or buy a larger tank.