I'm still a bit confused, so I'm just going to type out some thoughts and hopefully some will be applicable.
When the fish died, their deaths release ammonia into the water. The first step is to remove the fish and neutralize the ammonaia to prevent further death. This can be done with an ammonia binder, such as Prime or through large water changes to dilute the ammonia. There is still the question of what caused the clowns to die as that could still affect the other inhabitants, so this is important to try and figure out. It's also important to regular test the ammonia levels at this point. Lastly, it's important that any large water changes are not done too haphazardly to prevent shock, matching temperature and salinity for starters.
Almost all of your biological filtering is handed with the live rock. Unless ammonia gets WAY too high, they will continue to do just fine and process that ammonia into nitrate. Therefore, there is no reason to clean the live rock, nor to add any live rock. You can choose what to do with the filter media- most times this doesn't add too much to filtration in most systems. There are exceptions, but assuming you have a non trivial amount of live rock in your display, you need not worry about keeping that media, chucking it, etc.
It is always possible mixing the sand released hydrogen sulfide if your tank were old enough and the sand was deep enough to cause issues. This wouldn't be my first guess unless these conditions apply to you. Even then, I would still investigate other more likely culprits.
Any cleaning is likely not going to make much of a difference- the best things you can do now is to A) investigate the source of the crash, B) monitor your ammonia levels, C) increase aeration in the tank as a bacterial bloom secondary to the crash can quickly deplete oxygen, D) detoxify the ammonia currently in the tank, and E) maintain stable parameters during this- not to change too many things too quickly that aren't directly affecting the health of the animals.
