Bro, just my 2 cents...
If you do water change let say a bucket of Saltwater.
> Take half a bucket of aquarium water and use this to wash the dead sand.
> Then add the dead sand to the aquarium ( by this time your water will become cloudy due to the sandbed disturbance)
> Take another half a bucket of aquarium water (with this can remove some of the floating detritus)
> Fill in the aquarium with the newly mix 1 bucket saltwater.
A thing to note: the level of saltwater in the tank will increase due to the added volume of sand (need to consider any issue of saltwater overflow in the tank)