Help sick fish reoccurring?

I'm having a problem with my fish over and over. i have had a lot of fish in the past. but they all seem to die (yes i know they aren't immortal) and it always seems to happen the same way. they start to float more than swim, and wobble. they can't keep themselves upright. and then evenutally they sink to the bottom on their side. i use to work in a fish store, a long time ago, and i have never been able to answer this. maybe someone out there knows, or has some kind of idea. thank you.