Just keep in mind, that if you've had him for a year, and this is the first time you can recall that's he's seemingly lost his appetite for more than two or three days - - Weight loss, coupled with no appetite isn't the only indicator that a fish is ill. There are actually several afflictions in which you won't notice any significant weight loss until the sickness has progressed to the point where death is more probable than recovery. Don't freak out though, I'm not saying your lion is in fact sick, I just want you to understand that weight shouldn't be the only thing you look at when an appetite is lost, in order to determine severity of the issue.
Is there anything else, anything at all that he's been doing differently, or any changes in behavior, breathing, swimming, social habits?