Sorry to hear you have lost most of your fish. Last week I lost most of my malawi cichlids (see my thread 'Disaster' now on around page 3 or 4 of the emergencies forum).
My tank's water parameters were absolutely fine for ages - the tank had been running since last November with no problems - the fish had all been moved into that tank from another in which they had lived for a couple of years. Some of the fish were aged around 4 years and had never been sick. In fact none of them (except one) had ever been ill. I left them at around 5pm one evening, all looking fine, and by 9.30am the next morning all the adults were dead except one, plus one syno catfish. Only one adult cichlid, one adult syno and several small baby cichlids survived.
The airline had come undone and a kink was discovered in one of the filter hoses. I don't know how either happened. After removing the dead fish I tested the water and the nitrite, which was usually zero was sky high and ammonia, which was always zero, had risen also.
I was also overstocked, though not intentionally and only temporarily, but as someone told me also - when something goes wrong in an overstocked tank it goes wrong very badly.
It is a very hard lesson and I know how you feel.
