The sand is dead from the start, and will be colonized by bacteria that will cycle organic material back into available nutrients for plants.
You do not need capsules at all, ever. Dosing the water column with those compounds I mentioned is all that is needed. Keep the sand as clean as possible.
You can also use a product called Thrive. It’s a liquid that you simply dose the water with following the directions on the bottle.
Are you planning on using 100% sand, or what is your substrate you have already purchased?
What filter are you planning on using? That will help determine if you have enough flow for the tank or not. Typically you want a rated flow of 10x the tank volume per hour which will (in reality) be around 4-5x actual tank turnover per hour (because manufactures are poor advisors, but masters in deception).