Two of my little fish died in the last week, about 4 days apart. Before
they dies they were bent in 'u' shapes and then they just skulked away
to die. They looked like they had 'internal bleeding' if that makes
sense. One of he fish was spending a lot of time on the bottom of the
tank before it got really sick, so i medicated the tank. That didnt
help though. The other fish died suddenly. Does anyone have any idea
what that could be and what would have caused it? More important to
know what would have caused it so I can prevent it again.

My other main question is about my remaining fish. It seems fine and
healthy now but I'm worried it will die the same way as the other
two... do parasites live in the water? I just dont know what to do
about the water because obviously i don't want to put compltely new
water in the tank, but i don't know if there is some 'disease' in the
water? So i am reluctant to put any new fish in there as well...

All advice would be appreciated.