Hey everyone,
Just a quick note regarding a common occurring issue. I get a number of pm's regarding unexplained fish death, illness etc. I find this is common in the fish disease section as well.
First off, poorly buffered water will cause this problem. I wont beat this to death but if your fish cannot balance their fluid and mineral content, they will get sick. This does not apply to a few soft water fish as much but the majority will suffer.
The second popular cause of illness can be caused by too large a water change. On several occasions in both FW and SW, hobbyists have performed too large a water change, over 50%. Changes over 50% can cause major stress to your fish and their environment. Smaller more frequent water changes are always more beneficial and safer. Many fish can show signs of illness from this but not actually have a disease.

Just something to think about.