Well, this is why it is so important to use a QT tank! The one security is knowing that if it was some type of disease you have not exposed your main tanks. Yeah for that!
NOW FIRST ***My disclaimer: This applies only after all regular fish-keeping protocols are being done 100%!!!! Tank fully cycled, religious water change schedule, water always conditioned properly - w/ real measuring tools used - no guesstimates on dosing, regular filter cleaning, etc., etc. * * *
After several years of fish-keeping I've now come to accept and not beat myself up that not all fish make it. And sometimes you'll just never know why. Sometimes you can know and pinpoint what makes a fish sick and die. Sometimes it is just impossible. You may just not have enough info available to you, ie., longer and detailed history of behavior, origins, shipping methods; a larger population to observe/compare; more diagnostic tools - microscope. It may have been a disease - bacteria or fungus; or perhaps parasites - internal, ie., intestinal, blood, or gill flukes., etc., etc. Sometimes fish can be injured during mishaps in shipping, ie., ammonia burn. Sometimes the fish is just weaker stock and/or has been thru too many changes. I think stress does take a bigger toll than we consider.
I've found that cichlids in particular can be very stressed by moving around. I personally think it is because they are so very smart! I've moved angelfish that I've had for months, from one tank to another in my home, same water parameter and very well planted. Still they hide, sulk and go off feeding for a few weeks. Had extreme problems w/ Laetacara dorsigera when I move them around. They've gone from adorable begging puppy-dogs to completely hiding when I enter the room.
I'm just sharing these things because often when there's problems with fish we think it must be bad water or disease, or bad bugs, etc., and forget to take into account the more subtle stresses fish experience.
Even for humans, moving is one of the top 4 stresses you go thru in life. And we know and understand the whys, whats and wherefores. Fish can be anxious because their entire environment has changed, and their 'homies' are now gone!!! Gees, they just don't know what's up! Fortunately for us fish keepers, most of the fish handle the change and do adapt quite well into their new homes.
When I experience losses (either mysterious or determined) then I tell myself to remember: that's why keeping fish is a hobby - Fish keeping. A hobby with a big learning curve, as well. It's not quite just like keeping a pet dog or pet cat! All in all, in spite of being in our homes fish still retain their 'secret lives' and mysterious other-worldliness! I also vow to remain dedicated to best husbandry practices and continue growing my knowledge. Perhaps this is also one of the things I enjoy the most about fish keeping - because there is so much to learn I've yet to be bored.