Illness/Sickness: Please help axolotl tail

Okay so I have a Axolotl here, she was a rescue. I have 2 in the tank. I’ve always done water changes almost every 3 days, and I pick up the poop whenever I notice it. They where never perfect, the previous owners where terriable. But I still love them, I’ve had them for 6-7 months. Water is at 65-68 degrees right now. I noticed a small decay (no idea what it is) on my white Axolotl. I moved her to the fridge. It has really changed with in the time she has been in there. I do daily water changes on the tub, she lost all color the first week. But has now gotten it all back. She starting to look better. I don’t know if the decay was maybe the tail being caught on the drift wood that was in the tank, and it was regrowing, or if maybe her brother took a bite. It hasn’t spread, and doesn’t look like fin rot.

Info
20 Gallon long,
2 small Axolotl, 1 male, 1 female,
Decors terra-cotta pots, (previously had drift wood)
Medium sponge filter, 2x week wager changes,
Bare bottom, with light sand, for grip.
Light only used when doing maintence.
65-68 DEGREES

Okay, so many details needed.

First pics please. Hard to research it without seeing it.

Second, water parameters; adding an axolotl drastically increases the bioload, so I expect your beneficial bacteria cant keep up. Can you provide ammonia, nitrite and nitrates? And because color loss can indicate annemia, pH wouldnt hurt either.

The tank was properly cycled, and has been running for months, around 6-7.
I tested the water and it was perfect.

Unfortunately with the description you provided and no pic, I really am not certain. "Decay" could be, as you suggested, a bite or tear, but could also be bacterial problem or even a fungal one. Without a pic, I would just say as long as she is on the mend and it doesnt happen again, I wouldnt be too concerned. Just keep an eye on it.

I would also like to see a photo of you can provide one!