A couple of things, you can't have zero nitrates in a cycled tank. How long has the tank been set up? You said you unplugged the heater, what is the current temperature in the tank? You also said after you unplugged the heater there were no additional deaths over night, what was the temp before unplugging the tank? Do you have any pictures of the fish? The sore on the Discus gill? Anything I can see? How were the fish darting around and for how long before they died? This is a difficult issue to pin down there isn't too much specific info.
The API Master Test Kit is not sensitive enough to register anything, in my experience. I -know- there is some level of nitrates. However no test chemical based or test-strip based test registers the levels of nitrates in my tanks.
So, unless all tests, done by me AND the store are incorrect the simple truth is (as far as I can tell) my filter is functioning very well.
The tank has been setup for a little over 2 years.
The current temperature in the tank is 76.6F
Correct. I unplugged the heater last night and there were not additional deaths over night. This remained true until this evening when I discovered a small angel has died.
The temp before unplugging the heater was 81F
The fish (aside from the Dicus who became VERY visably sick) behaved normally as far as I can tell.
Is there any specific behavior you're looking for? They eat. They move, they're not lethargic.
This is a difficult issue to pin down there isn't too much specific info.
Yes! Perhaps you see now why I am so upset and annoyed! There is NO apparent reason for these fish to just start dropping dead!
There is something wrong...and as far as I can tell, thus far, it's not water quality. Everyone wants to point the finger at water quality but it seems that, that's just the safest bet! The easist thing to point ones finger at, but it's simply not the problem here.
I do not have pictures of the discus. I can/will obtain and post some picture of current survivors.