I've been keeping fish a long time, so i've had quite a lot die.
Most of mine died of unknown causes and/or old age. Though I had a few goldfish, including one who was 17 years old who died of fish TB. I've also had the odd case of dropsy. And I killed a three by overdosing my CO2 while I was on holiday.
My most memorable fish, who eventually died of old age, had a very nasty, unknown disease (that another one died of). It started in the tail, like fin rot, then ate it's way up the fish! So I'd have, half a fish, swimming about (not very well) with bones sticking out! I saved one, and it's tail completely grew back. I used anti internal bacteria meds, plus melafix.
I've had a few die of internal tumours aswell.
At the moment my black widow tetras are slowly dying off. They are very old though (I've had them since early 2004). One by one they're becoming thinner and weaker and then they die. This has been going on nearly a year. I'm pretty sure it's old age, because none of the other fish seem affected. I only have three left now, and I started with 10.
I also lost a lot of fish in a tragic moving house incident. I never checked the thermostat on the heater in my holding tank (the fish were in there while the big tank heated up). It went up over 30 degrees C and most of the fish in there died.
