i just bought a GT at petco the other day at the store he was really bright its tail looked almost yellow well when they guy caught him and but him into the bag his color got darker and now he's in my tank and is really dark i had my water tested and everything checked out ok he's eating and he's in there by himself would it be that my gravel is black and he's adjusting to it? or is he stressed out? is there anything i can do about it?