I'm not sure of the brand, Sorry its a fish at my work and I'm new to the whole fish thing. Its a food that is supposed to help the colors on him. I'm sorry that I cannot be more specific for you. He's been on the same food for almost two years.
by left fin do you mean his pectoral fin (the little ones he flaps and uses to move around?) does it look like it's continuing to deteriorate?
I've heard of fish biting their tails, but i'm also concerned about fin rot. How often do you change his water? If his water conditions aren't optimal, fin rot is a possibility and you'd need to treat that, depending on its severity. If it's not too bad, clean water and aquarium salts. If it's getting bad, you may need to try a med like jungle fungus clear. how big is the tank? Other fish in it?
do you use salts in the water? generally, these fish are happiest and healthiest in 2.5 gallons or more. 4L is a bit small. I would recommend making sure you put 1/2 tsp of aquarium salt in to see if that heals it. Those fins are very important and if he loses them he'll have a hard time recovering. Try to keep the water as clean as possible and see if the salts and clean water help. (are you also using dechlorinator/stress coat?) it sounds like if you've had him for 2 years, you must be doing mostly the right things so keep an eye to see if it gets worse.