I have 4 Zebra Danios who are 6 years old. One has always been a little fatter than the others but has always acted normal. Active, good color and ate well. But just over the last few days he started getting noticeably fatter but still acted normal. Today he's not himself, hiding loosely in a plant, breathing heavy. He won't even come out for a snack. He used to be one of the first ones there. Is he sick? What do I do? Please help!