Never seen this before but seems like my endli's tail at the very edge about 1/4" is white. I think something chewed or suck on it. Endli is totally fine and tail as well. Weird.
So this fin rot happens overnight or it s slow progression phenomena? Night before when I fed them endli was in perfect condition and next morning it has white in the tail
I strongly suspect a chew effort by fellow mate but I will be throwing in some medication after 90% water change just to make sure
If you have pics it'd help. Have you added or changed anything in the past few days? I would definitely stay alert if it is fin rot, but not jump to any sort of panic.