If the tissue is falling apart and "rotting" cut away the bad tissue and a little healthy tissue with it using a clean razor blade, then dip in a coral dip for 15 minutes. You can even apply a bit of the dip directly to the cut or injured area for a minute, and then dip normally.
Give the leather Moderate flow to help keep the area free of slime.
Leathers are very hardy and adaptable, but temp should be kept stable, I've kept them anywhere between 74-82 degrees, as long as it doesn't fluctuate, they should be fine. Also make sure it is getting decent water movement. I think many people underestimate the amount of flow leathers like.