I have had Craig my puffer for 4 months, and he is about 6 inches long. Last Thursday I did a normal water change and cleaned out some of his clam shells. The next day he was hanging out at the bottom of the tank were he likes to put his shells when he's done. Since he didn't come to great me when I got home from work, I immediately tested the water & all the levels were good. He looked ok & still ate some, though not as much as normal, so I thought he was upset because I cleaned his area out. Two days later he was starting to act himself again, and ate a shrimp. But the next day he was at the bottom of the tank again. This time he was breathing heavy, his skin is discolored, holding his fins in, and started looking bloated. He is in a 150 gal tank, with a clown, lawnmower blenny, pajama cardinal, and a lunar wrasse. I have not added anything to the tank in over 2 months. I bought the set up used and ran it for a month with the cardinal, clown & blenny since they came with the tank. None of the other fish seem to be having a problem.