1. Does anyone know the pH of pool filter sand?
It is an inert substrate, so it won't affect the aquarium's water chemistry either way. Many rocks, such as granite, are also inert.
Some substrates, like ADA Amazonia substrate (and things like driftwood and leaves) will acidify your water. Conversely, some substrates like crushed coral sand (and rocks such as limestone) will add carbonate hardness and make your water harder.
How hard or how soft they make your water, all depends on your tap water's composition to begin with. If it's very hard, it will have a lot of "resistance" and things like driftwood may hardly make any difference in the water chemistry. If it's very soft, like much of the tap water in NYC, it will make more of a difference. The stability of hard water is actually a blessing for most basic fishkeeping...if you're trying to breed fish it might be another story.
I already linked you this article before for that Rift Lake salt recipe, but if you didn't read the whole thing, I would encourage you to...it really helped me understand water chemistry better:
http://www.wetwebmedia.com/fwsubwebindex/fwh2oquality.htm
2. If it's not acidic, how am I supposed to grow Amazon species of plants in it?
Growing plants in pool filter sand is no different from growing plants in gravel, except for ONE thing - it is finer. Plant roots are delicate and root better in a fine substrate. Some plants will grow ok in gravel, but most would much rather have sand. The exception being Anubias, Java fern, etc. which don't have roots in the substrate.
You will need root tabs for heavy feeding plants like Amazon swords, these will need to be replaced regularly. As Jeta said, PFS is not the best substrate because it doesn't do anything really, except provide an anchor for roots. However it's way superior to gravel because most plants just can't root well in gravel.
Having a substrate with nutrients in it (PFS has none, of course) is helpful because it cuts down on having to do liquid dosing with fertilizers, which can lead to algae issues. That's why people are willing to pay top dollar for substrates like Fluorite and Eco Complete.
If you are trying to grow plants that only do well in soft water (many, if not most, are pretty adaptable) then the real issue is adjusting your tap water's parameters, either by mixing R/O water or rainwater with your tap water. When choosing a substrate for a planted tank, the effect on pH is not really the most important factor at all. Most don't have much effect at all, except for the ones specifically marketed that way (Amazonia, Cichlid Sand, etc.)
Here are some substrate reviews if you are ever considering a commercial substrate:
http://www.aquaticplantcentral.com/reviews/showcat.php/cat/8/page/
In my personal opinion, it's beneficial to invest in a commercial substrate in tanks less than 20-30 gallons. Tanks over that amount, 40+ gallons, it's worth it to try and come up with some DIY solutions, such as Jeta's substrate (Oil-Dri, very similar to a kitty litter substrate).
