is this spot on both sides? If so, can simply be stress caused as they stress easily. If on one side, can be bruising or vitamin deficiency. Assure it get a very proper diet:
-Spirulina brine shrimp
- LRS Herbivore diet
- mysis shrimp
- small plankton
- Nori seaweed basted with garlic extract
- Hikari Marine cuisine
- Formula 2 flake and frozen
- Hikari veggie marine
Add selcon vitamins to the foods 2-3X per week and on alternating days, garlic extract for stamina and immunity health