I have two betta's that have their own 5 gal tanks with filters and heaters. I did a partial water change last week. My blue CT is doing great, but the red CT is really stressed out. He hides behind the filter, and won't come out. He used to be all over the tank and was a joy to watch. He eats if I put the food near him. Occasionally you will see him leave his corner, swim up down and around, then go back to his hiding spot in the corner. Both tanks were refilled with the same pre treated water, and since the one is fine, I assume it is not the water that is the problem. Help?