Disturbing illness in fish

Question
I just found my African dwarf cichlid dead in his 25 gallon tank.  He lives alone, so no fish attacked him, but when I found him, his fins were deteriorated and his intestines were spilled out of his belly.  He also had swollen eyes and a bloated body, but I'm not sure if that is because of the disease or because of the decomposition.  Are there any diseases that can cause these problems?  I tested the water Monday, and nitrates, ammonia, nitrites, and pH were all within the normal limits.  This came on very suddenly.  He was fine this morning when I fed him, but when I gave him his evening meal (I feed my fish twice a day, small portions), he was dead.  Please tell me anything that might help and how to prevent this in the future.  I'm planning on draining and sanitizing the tank, refilling it and letting it cycle for a month or two before restocking, is this wise?

Answer
Hi Leah,
This is difficult to know what really happened to you pet.  I call this a Mystery Death.  He could have had internal parasites,or it could have been something wrong with his internal organs.  If he had had fungus, you would have noticed.  Fungus quickly eats away at fish, but you surely would have seen it before it got out of hand.  From the time you left to the time you got home, he may have died 5 minutes after you left, which left him in the water, so he quickly got worse, and looked awful by the time you came home.  Dead fish fill with water, and look terrible when not taken out right away.  He may have died of old age...many things are possible.  There was no reason for him to die, except for a disease that did not show.  Surprisingly he ate, sick fish do not usually eat.  It had to be something internal, and died suddenly like a heart attack in humans this can happen to fish.  Deaths like these cannot be prevented.  This wasn't something you did wrong.  Your water had the right chemistry, you fed him well, don't put the blame on yourself, he died, and nothing could have prevented it from happening.
It is a good idea to let your tank cycle for 6 weeks.  Test your water before putting a fish in it.  Make sure that your ammonia level is zero, nitrites zero, and that nitrates are safe.  Do 25% water changes every week once your tank has a fish in it.
I'm sorry you lost your little pet, we always feel bad when this happens, but some things cannot be prevented.
Lynda