If possible I guess go silicate free if you can find it. I just went to HD and got the ones they had, I believe it has silicates though. A pool supply store would also work, I've heard of people getting silicate free sand from them. From what I've read, here and other places is that the silicates can help algae grow, but I don't have an inordinate amount of algae in the 125g and never have. So not so sure if the silicate theory and algae is correct.
But I do know that seawater has silicates in it so youre not adding anything un-natural.
With such a small tank though you could probably get by with 2 bags of lfs
sand too. Not too too expensive. Like 50 bucks would probably be the total for "real" aquarium sand lol