Convicts are easy to breed, but growth is relatively slow compared to many. Also if they don't get eaten then they may set up shop and start to breed. You would be surprised what a pair of convicts can do to even the largest tankmates if they start to breed. I have at least 10 pairs in my lab for practice rearing fish for my students. We once used the extras to feed a pair of oscars. A small group didn't get eaten right away, and from that group a pair bobbed and weaved long enough to set up shop and lay a batch of eggs. When I looked in the tank one morning there was a pair of two inch convicts holding back two 12 inch oscars.
If that happened to your rays then those eyes could be easy targets.
You really want to feed something live breed swordtails in a kiddie pool outside. They breed fast and grow even faster. Also if you are willing to set up a larger tank for breeding nothing produces or grows like tilapia. A 1.6 colony can produce 3000 babies a month and the babies can get to 2 inches in 6 weeks easily if you feed them enough and keep them in a large enough system to grow out.