Originally Posted by Kumo
Did your shark have any tank mates with it? Have you added anyone new to the tank recently? If you've added someone new recently, it's possible one of them had some kind of infection (parasitic, bacterial, viral) and it got to your shark.
not knowing the actual setup he was in I'll say I'd venture it's not as likely from a new fish bringing disease. Rainbow sharks hardly do well with any fish and often kept in far too small a tank. Min tank would be 50g. That is enough to shorten the life span that is supposed to be7-8yrs or so.
Sorry for your loss Kittiesdragons!