I recently purchase a handful of fish but this one since I brought it home which has now been 4 days has been on the bottom of the tank and not eating. Every other fish is doing just fine. Today I woke up and it has a red dot on its scale. What can be wrong? I don't know if you can tell but the top fin is missing or is gone. All the water parameters are good. I did a 50% water change 2 days ago. I have a 20gallon. It seems to only move in the dark. And it hides all day. Under plants.