It is either fin rot or tail biting. Either way, I recommend doing 30% water changes every other day until it looks normal again. Then switch to weekly water changes to prevent fin rot from happening again. I also recommend feeding a couple frozen thawed bloodworms every other day to give him extra protein to grow back his fins. Also adding a teaspoon of aquarium salt per gallon could help. I hope your fish does well! If it makes you feel better, this is very common and either means your fish is biting his tail, got bit or ripped, or the previous water conditions were bad.