Toxin in the tank?

Question
We recently added two small red clawed crabs to our freshwater tank at the same time as a spotted plec.  We have done water tests and all the levels appear perfect i.e nitrate, ammonia, ph, nitrite.  However, we have lost four fish in the last two weeks. There are no signs of illness on the dead fish, both remaining plecs are docile and not moving much, but we have noticed that one of the red clawed crabs has shed its shell in the tank.  Is it possible that this has released some sort of toxin that is killing the fish.  The other unusual thing is that the dead fish are absolutely solid when we remove them from the tank, in over fifteen years of fishkeeping I have never seen this.  Any help gratefully received.
H


Answer
Hi Helen,
There must be something wrong. But you don't think the four fish you lost died of old age do you?
It shouldn't be the crabs at all, I've never heard of anything bad about their molting. I have kept Fiddler Crabs with fish and never had trouble. If you are certain its not water quality issues and the test kits aren't giving false readings (due to being outdated, residue in the test tubes, or the correct procedure not being followed) then it must be something else. The first thing I can think of is just old age. I don't imagine the crabs caught them. Many people try to lay lots of blame on crabs that they will catch any fish in your tank but its just not true. Fish are way to fast and crabs tend to rather avoid fish than go after them.

Have you changed anything different in the tank? Is the pH different? Did you add the bag water into the tank? (most likely you didn't I know) What is the temperature? Those four little fish may have just had something wrong internally we can never know.

The best thing you can do is keep a close watch on your other fish and take note of any symptoms or behaviors that may be "off"
I hope it doesn't happen anymore.

Best wishes and feel free to write with anymore questions!
Karen~