Fast the fish for a couple of days and then let them have frozen peas, which are thawed and a bit smashed. This is a type of fish laxative! you should feed them this laxative twice regularly till they saw improvement. If there is no improvement in a 5 day period, you can increase the temp of ur...
See, I have had some friends complain on similar issue with their fishes. As per the research i did then, it was classic swim bladder disorder. Probably, your baby shark is affected by that.
As you are saying that you have already lost one Endlar, so i think it can be an Ich, similar to salt grains. Better to get something from the pet shop for the treatment. Probably, they have something with themselves for this!