Rinse sand several times... It can come clean!
If it's live sand, use saltwater, possibly from a waterchange to rinse it.
I used a long automotive plastic funnel from PepBoys, If it isn't long enough, add poly tubing to the bottom.
A few cups of sand in the funnel, and then scoop a few cups of water from the tank in the funnel to wash it down.
move the funnel around as needed to distribute the sand around the bottom. You can even get in and around rocks this way.
2 scoops sand in funnel
2 scoops water
Repeat as needed