The potential exists for the Quetzal Cichlid ( Vieja synspila ) to have done it. While generally not too scrappy, some can be a little rough now and then. But really at the size you list it is almost impossible. Convicts are VERY tough! I will assume you measure or guess-timate your fish length from the nose to the farthest point on the middle of the caudel fin ( tail fin ) and if that is the case your male convict would have a body ( minus the tail length ) of just over 4 inches correct?
Just because it has not yet caught or perhaps consumed them thus far do not be fooled by the obvious choice seeming docile! The fire eel is an excellent candidate for the culprit! At 28 inches, this nocturnal predator may have either tried for the convict, or scrapped with and overpowered him.
Now it is entirely possible that your convict expired from non-conflict causes and was mouthed by other tank occupants. And as it was pointed out wisely by another poster, unless you see it, finding a confirmation would be difficult as best. But my money lies with the biggest fish and predator in your tank. Eels are HIGHLY intelligent, and can develope very dominant habits in a tank once they reach a large size and are fully comfortable in the tank. What you have in that tank sounds like a lot of food swimming around and IF the eel is to blame, you may be sure that when it is either hungry or annoyed enough there could be more dead or dissappearing fish in your tank. The male ( convict ) may simply have been a bit too big for it to swallow.
Do not mistake the fact that these eels eat a predomination of crustacians and insects at night with a lack of taste for fish. They are very adept hunters and if they are hungry enough or start to view smaller tank mates as palettable, they are quite capable of consuming fish. The good news is that with their spines and high dorsal fins the synodontis cats are most likely going to be safe. The bad news is that your Texas and Female convicts are the next likely targets IF it is the eel. Again without proof it is hard to say with absolute certainty. But if I were to wager on it, the eel would be my primary suspect. Barring that, the Quetzal Cichlid is a mild contender.
Take care and good luck with that! - Erik -
