Hi,
I usually dry the tank with hair dryer and remove the sand particles by vacuum cleaner to prevent scratching the walls.
Then fill the tank with water and add Whitex (Liquid Bleach) in it and leave it for a day.
After all the algae and stains were removed, emptying the tank, clean it with a piece of cotton cloth, refill the tank and add two to three additional dose of anti Chlorine and Chloramine to the water and use it safe. Here we use "Aquasafe" made by Tetra Company (German Co.) or "Aquatan" made by Sera Company (German too.)