Aquarium gravel has to be neutral. Tetras and Neon Tetras breed at a PH of 4.5, Cardinals at a PH of 5 to 5.5. Now Gouramis and Anabantids(surface breathers),(live breeder like Mollis, Guppies, etc need saline) like alkaline conditions.
Aquarium gravel under ideal conditions should be Quartz based gravel which is inert. Marble gravel/chips some put into aquariums but fish structure due to lime starts to fossilise. So which plants, Amazon or Cryptocrynes? Both acid lovers. Saggittaria, Vallessanaria, duck weed, water lettuce alkaline. If you want to propagate these plants then from submerged conditions take them to submersed.
Regards,
Arif.