I have a single discus that is kinda lethargic... doesnt seem to be eating recently.. been about a week... doesnt stay with the pack either... kinda isolates itself and swims quite slowly. and is quite skinny. I do water changes every day... and my water is amonia 0, nitrite 0, and nitrate 0. I have a planted tank, and do not fertilize... yet. i vary their diets but this one seems to have gotten sick? what is going on?