Do you mean attach it to the side of the desk?
No. You want a sheet of plywood or particle board that is large enough for the tank (or tanks) to sit on the sheet. The board can be an inch or two longer and wider, just so long as the tanks are completely supported by the board. The board should be 1/2 inch or thicker for good support. I had to do this for my 40g tank; I bought the cabinet stand when I moved, and for some reason the tank was 1/4 inch longer than the top of the stand. I wasn't taking any chances, so I got a sheet of plywood to lay on the top of the stand and the tank on that; it has a 1-inch extension all around the tank.
[I don't understand the question in post #29.]
On the sand, yes, there is quite a lot of dirt in play sand. I tend to rinse it about five times, and then in it goes. It is just dirt, and won't hurt anything, and it will settle out.