I just purchased two clownfish and they are in a 30 gallon tank. They've been doing fine for the past week or so, but a few days ago, I noticed that they both lost their appetite and have been huddling together in the bottom corner of the tank, breathing very heavily. I did a 20% water change last night and I tested the water today. Everything looks normal, but they are still really lethargic and not eating. Does anyone know what this could be?