I just read what you said about tail biting on another thread.
1) Tailbiting happens super fast
2) It just eats up the tail with no sign of other symptoms
3) If your water quality is good, it's probably biting, not rot.
4) If it looks shredded (between the rays) it's probably tailbiting.
This sounds like what happen. I checked on him last night around 1am and he was sleeping and looked fine, then this morning his fins looked awful!
He has no other symptoms, happy as can be (he acts like a puppy lol) checking out his new heater, watching my type this, eating great.
Water is great (I cleaned it out this morning just to be sure because I was so paranoid and added conditioner and salt)
Only shredded between the rays. The rays are still thick and long but the parts in between are gone. Only his longest fins are affected. Some of the rays are red (looks like blood)