Please Help can not figure out what is up with fish dying

Hi everyone

I have this issue on my hands where i have already gave up on searching for
an answer.

It all began with addition of neons and goldfish to an aquarium where i have
already had two eels and two arowanas.

Anyway two neons and goldfish died in about two week period where i noticed
that they had hard time coming up to the top to feed, and when they tried it
seemed like they needed a lot of effort to swim up. Eventually i have
discovered that they stay on the bottom laying on the side and were looking
around an breathing. Also i have noticed that their bellies became somewhat
big and got white color to them. In about a day or two after they stayed
motionless on the bottom they died. One thing i found interesting is that
when you touch them they would start swiming normally for an hour or two and
then go back to the bottom. The second shock whas about two month later when
all of a sudden i have discovered that one of my arowanas was swimming on
the bottom and had trouble swimming up, where it was usually at. Two days
later i saw her bottom up on the bottom of the aquarium and what was strange
it was looking around and breathing. I saw her swim couple of times but two
days later she died too. Today one of my eels died also, it was perfectly
healthy this morning, swimming around coming out for worms but at 12
midnight i found it on the bottom with it's belley up, and again it was
somewhat bigger that it should have been. With the eel i noticed that he
started to swim eratically prior to his death and he tried to come up to the
top for some reason.

Right now one of me new goldfish shows sympthoms of inability to swim up, it
speeds up and once it gets to the top of the aquarium it slows down and
drops to the bottom, the starts swimming up again.


If anyone has a clue what can cause those sudden deaths and inability for
the fish to swim up to feed i would greatly appreciate it.

Thank you very much

--
Sincerely

Alexey Semenov