Are you talking about that small split in the caudal fin?
If it is beyond the fin rays, into the body, it may be permanent, if only in the fin rays, it may grow back.
Raising the temp is not always a good idea.
Certain bacteria proliferate at temps above 82'F.
In the lab when trying to grow pathogens, the cultures are put in a 95'F (35'C) incubator. The bacteria that causes "duck lips" (columnaris( is one such pathogen.
Below is a split fin that never healed, because the split went beyond the fin rays, and into the flesh.