Most people recommend pool filter sand, but in looking on the HD site the same brand (Quikrete) has a 50lb bag of both pool filter and play sand, and the play sand is cheaper. Also, play sand from that company is just regular beach sand that's been rinsed for you, so it's natural.
If I were to purchase some sand for my builds (which I am planning), I'd go with the play sand. I'm not sure on the pavestone, so I can't say which brand to choose, but personally I'd go with Quikrete since they answered a question on their product page that asked about the sand's toxicity, as well as it being a more established brand.
As for the safety of your fish when you put the sand in there, they should be fine as long as you use something to get the sand to the bottom of the tank without allowing too much to stay in the water column. If you have pantyhose, you could fill it with the sand, lower it to the bottom of the tank, and gently spill the sand out, being careful to not agitate the sand where it'll float in the water column. Alternatively, you could use clean pvc pipe to transport the sand where you want it; just slowly dump the sand into the top opening of the pvc with the bottom opening near the bottom of the tank, and you shouldn't have any issues.
The only real problem with adding sand to an established aquarium is the sand lingering in the water column, effectively choking the light and starving any plants. The lingering sand can also get stuck in the gills of the fish if it's a fine particulate size, but if you follow the directions I outlined in the previous paragraph you should be fine
Hope this helps!
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