It won't grow back if it hasn't yet, probably like that since birth. There is a lot of inbreeding involved in some varieties of angels, koi are one of those varieties.
This is the part many people don't like to hear, but it needs to be said; fish with defects need to be culled by the breeder. It does nothing for aquatics to put defective fish on the market, some times there are unseen internal defects along with the visible external defects. These internal defects make them more prone to disease, which shortens their life and could be a problem for tankmates when the concentration of bacteria rises from the ill fish.
Any responsible breeder of ornimental tropical fiah will cull defective or runt fish, and not breed fish that have defects or produce spawns with an abnormally high rate of defects.
