I second what Elliot said.
In one of my tanks I'm using BD trading sand (Aquarium Sand 25kg). It doesn't look anything like it does in the photo. In person it's very very pale. I've got it in a tank with a 100cm x 40cm base and there's more than enough. If you've never used that website before you'll get 5% off your first order (and free delivery as well, I think).
This sand is very fine and great for ACF substrate but expensive if you have a larger tank: Caribsea Tropical White Sand 9kg
I've also used the sand from Pets at Home - £11 for 8kg, which is more than enough for a 2ft tank, and never had any problems with it.