I decided to try a 10g shrimp tank. I intend on using organic miracle grow and a cap. The petsmart that is only one block from me has CaribSea FloraMax I was considering for the cap. I am concerned it may release too much Ca and raise the GH too much. I initially looked at pool sand at lowes but it had a warning about silica so either it doesn't matter or was the wrong type.
If you're already spending $10-$20 for substrate, why not buy something that's nutrient-rich or that has the ability to absorb and release nutrients slowly over time? For a 10gal, that's relatively cheap to do. You could even get Aquasoil Amazonia for about $30, depending upon the shrimp you hope to keep.
I just bought a huge bag of "play sand" at Home Depot for less than $5 as I recall. It is incredibly fine. I can't attest to it being inert since I have yet to add water to the tank. I used the sand as a cap and as part of the substrate.