I have 5 large silver dollars in a 55 gallon tank. I have had them about 6 years. They have always eaten well and I have not changed anything that I have done in the past. Just in the last week I feed them and they hide in the corner and when they venture out the food just floats by there face and just goes to the bottom. They use to go crazy with the food. The food sits on the bottom and I believe they eat some later but not much. Any ideas???