Hi Pinky, I just got a bag of kids playsand, Sainsbury's do big bags of it for cheap. Before you stick it in the tank though put it in a bucket or something and rinse it loads to get as much of the dirt out as possible. I also found the playsand had very small bits of gravel in it so try and remove that if you find any.
It does look a little cloudy once you put it into the tank but that will settle, just make sure it's very well rinsed before it goes in. :happy: