I used Argos play sand too. £2.98 for 15kg's. I bought 2 bags for my 125 litre tank and ended up with a bucket full left that i have donated to the sister in law. Easy to wash, cheap, and although it looks a very unappealing colour when in the bucket, it is very nice when in the tank with the lights on it. I am very pleased with it, but i guess if you wanted a dark sand substrate you would have to go for the proper stuff? Unsure if any shops sell black sand for anything?