Is it possible there are too many fish in your tank? I by no means know what I am talking about, however I have a 25 gal tank and in it I have 1 Pleco and 6 misc tetras and barbs (like I said, I am a beginner)...if I so much as put a new fish of similar kind in the tank, something will die.
Grasping at straws, but you never know
I found some useful info here...
http://www.thetropicaltank.co.uk/