Yep I just went and asked a pool place (that I saw from the motorway lol), to wash it, simply fill half a bucket, and run it under a tap whilst youve got your hands in churning the sand around, then carefully empty the water, and repeat 3 more times, then when it comes to adding water to the tank there will be next to no mess!
It maybe alters the ph a bit, previous tanks (without sand) have been 6.8-7.0, and this tank sits between 7.0 and 7.2, and it may not even be the sand causing that, even so, its minimal.
I hope this helps