For some reason, African Cichlids in general seem to be really prone to internal parasitic infestations. I don't know why this is, but I see it in them more than any other group of fishes, including new worlds. I wonder if it has to do with the way they're raised, or what.
I had the same problem in several mixed mbuna and peacock tanks, and had to ask a lot of people to figure out what the cause was. Fish would, as you described, be fine one day and then over a matter of days or weeks, their belly would become increasing and frighteningly concave. I had one fish that looked someone took a half-dollar shaped bite out of his belly.
The thing about internal parasites is that virtually all animals have them, and they're not really a problem. Remember, a good parasite doesn't cause so much harm that its host is ever put into danger. However, when the animal is stressed, its immune system cannot respond, and the parasites can increase in number beyond the "healthy" level, causing severe problems. So, something stressed your fish out -- he got chased by another fish, his favourite team lost, who knows. Whatever. The parasite population in him was able to explode.
I've had really good success using a goat dewormer in tanks to get rid of them. It's a HECK of a lot cheaper than Prazi. You can also find other products with praziquentenal in them, though the active ingredient in the goat dewormer is fenbenazole. You can pick up a bottle of it under the brand name of Safe-Guard for something like $20 at Tractor Supply. Use it at about a drop per gallon -- usually one dose. It turns the water uber cloudy, though.
Fenbenazole is apparently very safe ... I've accidentally overdosed it and not had any ill effects.
I have about a gallon of it out in my car. Send me a PM before the next meet, and I'll bring you some.
