SandyCheeks
Aquarium Advice Freak
-"Do you swish your filter media in conditioned water on a regular basis?"
i was never told to do anything like that, you mean the actual carbon filter?
Yep, the filter media should be rinsed periodically- mine is a sponge, not the actual carbon filter. For those of us on city water, we clean our filters in old aquarium water after a water change; the water that is chlorine free and wont kill the good bacteria. Do you add chlorination to the well water? Either way, you can add dechlorinator to a bucket of water and swish your filter media in that. I think it would be a good idea to give it a good rinsing if it's never been cleaned.
This IS a mystery. Your other tank tests normal? Are both tanks the same age? Did you do a recent water change on the other tank also?
I am wondering if there could be additional bacteria in the well water combining with the bacteria in the tank and having a hay day on your filter media that's never been rinsed, producing high Nitrates.
If it were me, I would do a water change with something other than well water, just to get the Nitrates down from this dangerous level before it hurts your fish. I would try using a friends city water or store bought - verify it has the close to the same ph first. (I have used store bought drinking water before. Just the regular drinking water, not distilled.) And remember to use a good water conditioner/ dechlorinator. I know it's a pain, but I am worried about your fish. And that way you can rule out anything in the well water if the Nitrates spike again.