I’d run some carbon, and stop doing lots of drastic waterchanges, it sounds like you’ve probably done plenty.
Might be easiest to catch the tang at night, with a small net and a flashlight. You blind him with the flashlight and scoop him up. It might take a few tries. If that doesn’t work, I highly recommend getting a trap, it will be useful in the future.
FWIW I doubt the Reeses Peanut Butter cup is the culprit. Who knows what they did though.
Running a “Poly Filter” might be helpful they absorb all kinds of stuff.