That definitely doesn't look like the play sand we can get at the home improvement stores. Our's isn't white but it's light tan. That looks to have a lot of dirt in it. Try Lowe's or Home Depot. They should have Quikrete or some brand such as that in 50 or 60 pound bags.
I dunno, I'll try looking. We live in a small town and gasoline prices are really high so I can only go to chain stores every once and a while. It's for a betta growout tank, but now that I think about it, it's probably better to leave it bare bottomed?
But for your uses ill throw a vote in "bare bottom" box.
Its funny that's a UK brand of play sand but here in the states we have almost that EXACT same style bag, little kid and all. sold at home depot of course.
EDIT: I almost forgot / pushed it out of my head due to trauma....
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should you decide to go with a fine white sand like that and all...