Hmm...I'm really stumped on this one for now, the only thing i can think of, depending on the fishes weight, is that it was either internal bacteria or internal parasites- internal bacteria can kill quickly and sometimes cause almost seemingly random deaths in the tank, while internal parasites can waste away the fish even if its appetite is normal (internal bacteria can also cause wasting away); to make sure that their weight was not an issue before they died, here is a link with some pics of fat and healthy bala/silver sharks pics for comparison;
http
/www.fishforums.net/index.php?showtopic=123786
Did yours look a bit underweight at all in comparison to these bala sharks

? Many bala sharks sold at lfs are underweight so sometimes it is hard to know wether yours are the right weight or not is the only sharks you have known were ones supplied by your lfs.
ps: i am glad i have been of some help though in other area's of yoru fish keeping, good luch with the tank and i hope the cycle goes smoothly too

.