Hello Mark, 
I have bread convicts in a 90 gal. tank with 10 inch oscars.  The 
parents defended thier territory well, and the eggs most always hatched. 
The fry did well for a week or so before they started to disappear.  So I 
don't feel your problem is with other fish in the same tank.  I think 
the eggs are being eaten by the parents themselves.  The reason could 
be water quality, immature male, or habitat.  I would first check the 
water, then I would put a plastic cup in the tank for them to make a 
home (so to speak), and watch what happens.  Convicts are prolific 
breaders so you shouldn't have to wait too long.  To be of more help I 
would have to have more info IE:tank size, PH, O2 level, so on...I hope 
this has been of some help, and good luck. 
 
 
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