Convicts will kill rams, if you want a tank mate I'd go with a sev for sure. I'm surprised your convict didn't kill all those tetras. Convicts are just as aggressive as red devils, they're just not as big. Severums get kind of big though. Keyholes, and festivum would be good tankmates but the con might bully them too. I wouldn't add a jewel, they don't get along well with convicts. A firemouth would work but you could end up with hybrids. It's up to you but your best best is a severum if that's all you can find locally.
