there is something wrong with my dwarf puffer. I came home from work today and he seems extra fat on his lower end and is not eating. I don't know what is wrong. I did a 20 % water change with a quick start this morning and he seemed fine. he is in a 40 g breeder with a few different tetras and other fish. He has seemed fine until now but now I'm not sure. can someone please help. The ph and nitrates are normal.