Going off what we know from your other thread, it' most likely the tank was not cycled properly and that's what killed the fish. If it was a disease then what you need to do is remove all the water and disinfect everything just to be on the safe side. Then you need to decide if you want to fish in or fishless cycle the tank.
You can use a bleach solution. One part (plain) bleach and 9 parts water to wash everything. You then need to rinse it in 3x dosed Prime and water. Let everything dry completely then you know that everything is clean and ready to start again.