Btw- tail bitiing usually looks like little U's bitten out...Clean slits or clean cuts in the fin are more likley due to a plant or decoration hurting him.
Some fish are just derp.
Some fish are REALLY derp.
He might be a chronic tail biter, I had one and I loathed him for it DX
If you find this is a chronic problem, just make a point to keep the water clean. Even if it is fin rot, its probably from your tank not being cleaned enough (25% isnt enough- even if ammonia is low, the bottom should have the nasties sucked out!) , so just make a point to frquently change the water, you wont even need salt if the waters clean enough.
Also, if you really do suspect fin rot, put in a bubbler if you have one, but dont set it too high. It helps stave oiff bacteria.