IME with purigen you can't overdose it. I'm not currently using it in any of my tanks right now, but that's because they already have 0 nitrate and phosphate. So I'd start looking for other causes for you fish's behavior, even if they're indirectly related to purigen.
How have your parameters changed since adding it? I know your nitrogen levels have decreased, but how about your pH? If you have reduced nitrification, your tank is consuming less carbonate and your pH would stay higher. Has your pH always been 8.2? That's very high for most tropical fish, and especially a jaguar cat. I believe they're Amazonian, meaning they'd prefer slightly acidic water. You're in the reef keeping pH territory!
What's your KH & GH? I'd be willing to bet your KH is at least 150ppm with that pH.
Since you have it in a reactor, why not try shutting the pump off and run the system without it for a while to see if things return to normal? Maybe this might provide some answers if you can't find any here.