Swim Bladder Disorder Contagious?

Last weekend I picked up some API Root Tabs, two moneywort plants, two neon tetras and 1 glowlight tetra. They are all in my 10g tank now with the original 4 glowlight tetras and 3 neon tetras bringing the total to 5 and 5. I also have a few RCS in there. This was Sunday I think. The original fish in there have been troopers and had no problems for a few months.

Late Monday night one of the neons was swimming erratically, rolling and floating funny. The next morning, tuesday, a neon and a glowlight were laying dead in the tank. Everything else seemed normal and fine. Did a 40% water change at lunch on Tuesday. Wednesday afternoon one of the glowlights started swimming erratically and died in a matter of minutes. Today the same thing happened to one of the neons. None of these fish seem to show any sign, at least that I can tell, that something is wrong until they go.

I've done another water change today and we are down to 3 and 3 fish. One of the RCS also kicked the bucket but they are pretty new so no clue if that's related to this or not. Since they are displaying no real symptoms prior to going crazy and dieing is this swim bladder disease/disorder? If so, is it most likely bacterial since they seem to be succumbing at different times?

Water is 0 Ammonia, 0 Nitrite, 5ppm Nitrate, temp 76F, Ph 8. The PH has always been that high.

Any suggestions? Going to get the wife to pick up API Melafix in case it is a bacteria issue.

Thanls for the responses.

No other symptoms that I can tell. Everything's fine and 15 min later they are swooping and rolling and floating nose or tail up then they die.

Of course, this morning everything looks fine. Will see what happens at lunch and tonight.

How would I test for something like a chloramine slug or anything else for that matter? Also what would be the fix?

EDIT: Honestly it's the sort of thing that looks like they are eating something that causes them to fail miserably. It's very sudden and fast and as I said, 6 of them are still alive doing fine. I would expect if it was something like a Chloramine slug, which I honestly don't know much about, or something in the water it would affect all of the fish similarly as opposed to one or two being affected at random times.

Ok, thanks. Everything seems to be fine in the mystery tank of death at lunch today. Everyone's happy and playing around. Hopefully that will last for at least a day if not indefinitely.

I normally use prime so it should be taken care of. I'll go ahead and dose the tank tonight just to be on the safe side though.

No way to tell. I know at least 1 of the old fish died since there are only 3 glowlights now. There's no telling on the neons. I'm inclined to think the new ones died since they seemed smaller.

I figured I would chance it again and no QT. Next time I'm QTing the new ones for at least a little while.