One thing I have noticed with this group of fish (as you go forward) is the name : Mikrogeophagus. Mini-eartheater in English. They like to sift food from fine sand, and are very slow eaters.
I agree with everyone else that it could be worms, but if it is, all your fish have them. Usually, they keep them in check, but if one is being dominated by the other, the stress will allow a minor infestation to run wild. So you likely have 2 issues - the usual chronic gutworm issue from farms (since you have had them so long, nematodes like Camallanus are less likely, but still possible). I'd find whatever local brand of praziquantel I could and treat the tank. If it doesn't work after a month, then consider the harder to find anti-nematode meds.
When you return sick guy to the tank, the problem will return. 2 and 3 are bad M. altispinosus numbers. In a larger group of a Cichlid like Bolivians (this doesn't work for more territorial Apistos, for example) you can go up to 6 or so. That way, any aggression is spread around, and no one gets keyed in on.
I agree with everyone else that it could be worms, but if it is, all your fish have them. Usually, they keep them in check, but if one is being dominated by the other, the stress will allow a minor infestation to run wild. So you likely have 2 issues - the usual chronic gutworm issue from farms (since you have had them so long, nematodes like Camallanus are less likely, but still possible). I'd find whatever local brand of praziquantel I could and treat the tank. If it doesn't work after a month, then consider the harder to find anti-nematode meds.
When you return sick guy to the tank, the problem will return. 2 and 3 are bad M. altispinosus numbers. In a larger group of a Cichlid like Bolivians (this doesn't work for more territorial Apistos, for example) you can go up to 6 or so. That way, any aggression is spread around, and no one gets keyed in on.
