I am sorry to hear about the fish, that is unfortuntate. What I would recommend is reading up on material regarding how to care for a smaller saltwater aquarium. The main things you need to be concerned about is stable parameters, a good source of water, a light fixture, and a simple filtration system to keep the water circulating. It seems like you have a filtration system, so that is one thing down. In terms of stable water parameters your concerns now are ammonia, nitrite, nitrate, and salinity (assuming you don't have coral in there). Since this tank is established I am sure that it is already cycled, but the stirring of the sand probably caused a spike in ammonia. Preferabbly you want no ammonia, no nitrite, and some nitrates. The one that will change the quickest if not monitored is salinity, as you have to keep adding freshwater as it evaporates. Best of luck to you going forward.