Help. Sucker Fish Starting to Float

[sammy]

Please help. I have a sucker fish that was fine when I left this morning, and now it is floating and seems to be close to death. I checked the levels and I have high nitrate levels, so I did a partial water change. What could have caused my fish to check out? I only have a 5 gallon tank, but I have four other fish in it. They have all been living together for approximately 2 or 3 months and have been fine until now. Any suggestions? Thanks.

 

[Menagerie]

There are a lot of unknowns here. How are the ammonia and nitrite?
What type of sucker fish was this? What were you feeding it?

 

[flipz]

Also what other fish are in there? 5 fish in a 5 gallon tank. Are they all large fish?

-brent