First off, let me just inform you now in case you did not already know -- silver-tipped sharks can grow to be quite large over time (12-18 inches max). A tank as small as yours is nowhere near big enough for them once they grow to their full adult size, so hopefully you have plans in the future to move them to a much larger tank.
As linksyz already asked, was your tank fully cycled before adding the fish? Have you been doing water tests to check your ammonia, nitrite, and nitrate levels to see that there are no traces of ammonia or nitrite in your water, and that your nitrates are kept at a reasonable/safe level through partial water changes on a regular basis? Gravel vacuuming is also a good habit to fall in to doing each time you do your partial water changes as well, as it will help remove a lot of the solid waste that falls down into your substrate and can cause you problems over time if it is allowed the chance to build up and really turn your water nasty.
As far as feeding goes, be sure not to over feed your fish, as that can easily cause you bacterial blooms (white, cloudy water) as well as ammonia and nitrite spikes in your water which can really stress your fish out and even kill them in more extreme situations. Also be sure that you have a heater in your tank to keep your water temperatures at the appropriate level for tropical fish and to ensure that the temperature remains stable rather than fluctuating all over the place.
Not sure what else to cover right now, but those are the basics, assuming you understand the nitrogen cycle and have properly set up your tank and are aware of the potential size that those fish can get. I'll wait to see your responses to my initial questions before I continue further with any more suggestions and recommendations.
Good luck!
