See "EI Light" under Estimative Index ! Once you get started you can study the Estimative index to fine tune your dosages for you particualr chemistry.

10- 20 Gallon Aquariums
+/- 1/8 tsp KNO3 (N) 3x a week
+/- 1/32 tsp KH2PO4 (P) 3x a week
+/- 1/4 tsp GH booster once a week(water change only)
+/- 1/32 tsp (2ml) Trace Elements 3x a week
50% weekly water change

Yes you will still need to add epsom salt and baking soda These are for KH and GH to buffer hardness. Not nutrients Per Se. Try Tom's "GH booster" as it's premixed and inexpensive.

Try to find onyx gravel, onyx sand, grey coast sand, or white calcite/Alabaster gravel "Commonly used for landscaping" but it would have to be a very fine sieve (2/4/6 mm )

Don't be discouraged by your LFS's inventory. Either have them special order goods for you or have it shipped in off the internet. This is where your local aquarium clubs may be able more than anyone for local resources. I order stuff from all over the world every week W/ very few problems. HTH. Prof M