Fish Dying One By One!

[cfmurray4]

Ya, So all my fish except my rainbow shark is alive, but barely. I put him in a separate bowl, and keeping it warm.. I have also noticed that their fins seem to be deteriorating away. Just another info I thought I would point out. So I took all the water out, cleaned my sand, filter, heater, and decor. I filled the tank back up with the sand, and keeping the rainbow shark in the bowl until he either dies or stays alive. I want to thank everyone for the info, this is so hard even though these are just fish it sucks to know that you killed your own fish!