Well,i don't know what you mean exactly by a sand-tank,but i think,its a tank for fishes same like for Lake Tanganjika or Lake Malawi.
When you put sand inside its very natural,and same like any other tank the dirt will come down to the ground.
Just use a hose(about 1 m) to such up the dirt every week! with refreshening minimum 25% of the water.
Like other people said,it is impossible to keep it 100% clean,but the waterchange will surely help!
I myself change every week 50% of mo the water in my 20 fishtanks.
the fish will surely appreciate it!
Lots of succes,Willy Bijker
Willy,Male,Holland,Dwarfs,West-Africa,Central-Amerika,South-America,Tanganjika,Author of articles,Give lectures to.