I bought a green mandarin right before Christmas. I was going out of town the next day and wanted to be able to keep an eye on him when I put him in the tank, so the store held him for a couple weeks until I got back. I didn't notice at the store, but he's had a sunken belly. He's active and picks around the rocks and sand throughout the day, and seems to eat a lot when I put food in the tank. How concerned should I be about his sunken belly? I don't think it's getting worse, but I'm not sure it's getting better, either. I'm wondering if progress is just going to be extremely slow because of them being such slow eaters.