Fish people - what disease is this?

[Pareeeee]

I've had two fish die in my aquarium in the past month, and both exhibited the same symptoms. I need to prevent this asap.

It's almost like a combination of pop-eye and dropsy. Both fish became slightly 'unstable' swimmers, their bodies bloated (but not to the extreme like with dropsy) and scales looked deteriorated. Eyes were enlarged, but not to the extent of pop-eye. The first fish was treated with Maracyn-Two (antibiotics) and raised aquarium temperatures, the second with Pimafix. Both fish died.

The second fish became sick soon after a water change. The illness did not affect its appetite until the very end.

Any suggestions would be appreciated. I cant seem to find any info online about these symptoms.