You have probably too little surface agitation. I can see a layer of biofilm, which is normal to accumulate among Salvinia older leaves, which then tend to rot and the rot spreads. You have to regularly weed out old and rotting leaves, then the healthy green ones I can see growing there on one of the pictures will take over.
But I also see some new leaves coming in quite lightly green. What type of fertilizer do you add? Could be potassium or iron deficiency additionally.
Try lowering the hours of light given to like 8-10 hours. More light = more nutrients used up. If it is a nutrient deficiency then lowering the light may help balance your tank. Or you may need to add more fertilizer based on the type of deficiency.