It isn't a waste, normally you would vacuum your tank after blowing off your rocks thoroughly, & while lots of waste is still floating in the water column, you can remove as much as you can. This removes salt also, so yes you need to make up a batch of DO/DI water first & salt mix to the same salinity as in your aquarium, adjusting the temperature & using a power head to mix the solution throughly, usually over night or at least a few hours until the water clears up, it is considered good house keeping. You can vacuum without having to do a water change though, I throw a 5 micron sock filter in the sump and siphon the main tank with a hose directed into the sock, the allows me to remove waste without actually changing out water, I usually do this between water changes.