I was just checking my fish after feeding them some brine shrimp. I noticed that our large silver dollar, Chubby, was behind the plants with belly on the gravel. He normally swims back and forth in the front. It looks like he has lost about half of his scales. We always look in the tank but this is the first we've seem any of our fish act strange. He has always been an aggressive eater, that's why he is the biggest. I checked the tank parameters 3 days ago, 0 ammonia, 0 nitrite, 40 nitrate. I did a 30% water change the day after the readings. I added a couple spotted pearl shrimp this week as well. The fish were fed live black worms yesterday for the first time. Now Chubby is resting on the bottom, never done that before. Please some advice. Thanks.