ok, another thought (you should see the smoke in here), have you searched the tank and decorations for a dead fish, or old food? done a gravel vac recently?
now i get to tell a story that you probably don't wanna hear.. lol. when we started keeping fish here at my house, my hubby did most (read ALL here) of the tank maintenance. when he left i had no idea where he was at with the tank maintenance, didn't know that he was doing HUGE water changes on my 150 because he couldn't look under a decoration to find the rotting food that a pleco had stolen and stashed away. i nearly killed all of my fish when that tank crashed a couple days later because i didn't realize that he had left me a problem.