That really sucks. I think everyone who owns fish eventually suffers from the tank of doom syndrome. And our first response is to blame ourselves. If you are dechlorinating the water, keeping the water parameters in check, and feeding well, then there just isn't much more you could have done.
If you haven't already eliminated these as possibilities:
1 new acqusition (fish, live plant, invert) that might have introduced disease
2 high ammonia, nitrite, nitrate
3 poisoning of some kind - metals, household cleaners & soaps, contaminated tank ornaments
4 heater malfunction
5 poor oxygenation (powerhead or air pump malfunction, tank got too warm)
6 any change in your municipal water supply, recent plumbing work that could have released toxic sediments
7 Fish nearing end of normal lifespan
8 viral infection - not much can be done for these - even for humans
9 just a crappy batch of fish, if a recent aquisition. Many fish come into the lfs
stressed and half-dead.