i noticed my goldfish (of nearly 6 years!!!) was swimming very low at the bottom of the tank for the past couple days or so. i didn't think much of it, until tonight, when i noticed he is now lying on his side on the bottom of the tank, and breathing, which looks as though is rapid and heavy, almost struggling. I don't know what to do, i changed his water by 50% in case his tank had become too dirty, but no change so far. I feel so sorry for the little thing just lying there. HELP, what can i do??