Following your threads over the last few months, I see two main problems: lack of quarantine, and housing species together that are either incompatible or need a larger tank to coexist.
Strict quarantine should be followed when you're mixing fish from all different sources if you want to eliminate issues. For instance, I just imported some fish from a very well known, very reputable, very respected source (a man who has discovered species, had species named after him, etc.) and the fish I purchased still brought a nasty bacterial infection with them. On their second day in quarantine they were shedding slime and showing patchy sides, very similar to columnaris. I went through 10 days of 100% water changes and $160 worth of Furan-2. The fish look great now and are eating well, but had I introduced them to my other stock, I would have had major problems. You never know what the fish you're bringing home have been exposed to, or what they're carrying, and so I always stress quarantine to people. No matter how reputable a source, they cannot always know what unseen pathogens and parasites the fish they sell may be harboring. I keep most of my new cichlids in quarantine for 6 weeks. It's best to do this in a separate room if you can, with separate hoses, nets, etc. for the QT tank. Wash your hands thoroughly even when you're just feeding tanks. It may sound extreme, but if you're investing a ton of money into fish (in this case, I spent $900 plus $200 in import permit fees/airline fees) it's a no brainer.
On the second point -- cichlids are cichlids. I have mixed some odd combinations in the past. Some work, some fail miserably. If they fail, always have a backup plan. I shuffle fish around if need be. If you look around your house and think, "I don't have anywhere to put [insert species here] if things go south in my main tank" then you may want to think twice about acquiring that species. Again, this is one of those things that's easier said than done. Sometimes you want to work with a species and you buy it without thinking ahead. I have been burned there, too. But now I look at it from the angle of, "Okay, am I just throwing away money/putting this fish at risk by making an impulse buy?" If you're working with large and potentially aggressive cichlids, tank size is going to dictate in most cases how tolerable they'll be of tank mates. Actually, scratch that -- the same can be said for most cichlids. Example: I have some wild Andinoacara latifrons that were beating the crap out of each other in a 55 gallon growout tank. I shuffled some stock around and moved them to my 150 and they instantly calmed down and went about their business. It's not that they weren't compatible, but they needed their space.
