I need help diagnosing a sick goldfish. I am in the process of quarantining it right now. The tank water is fine (zero ammonia, and nitrite, ph7). The fish's abdomen is bloated and he's rather lethargic. I thought it might be constipation. He ate a couple of peas yesterday, but does not appear to be getting better. Also, he seems to have lost his appetite, as of today. Any ideas on what it might be and how I can treat him? Thanks!