Maybe even take a water sample to a LFS (Big Al's or Pisces come to mind) and ask if they would do a battery of water tests.
When something's wrong, but everything tests OK, one possibility is bad test kits.
There are basically three possibilities here. 1) the water chemistry is off. The ones I would check first are pH, ammonia, nitrite. 2) there is a predator in the tank, or tankmates are not getting along and beating each other up. 3) disease or parasites.
You have to systematically go through the variables, and eliminate them one by one. As Sherlock Holmes said, once you eliminate the impossible, whatever remains (however improbable), must be the truth.
Good luck. And yes, a big water change couldn't hurt.
PS. What is your temperature in the tank, BTW? Is there a difference between daytime and nighttime temperature?
__________________
-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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