Which substrate is ideal for a planted fish tank?

There are TONS of different brands and types of aquarium substrates that are ideal for planted tanks. Many include substrate that is naturally inert, but has been sourced with a bunch of various nutrients to make it similar to soil, but in a gravel like format that can be recharged via dosing or root tabs. Other substrates can be just pool filter sand if dosing, or actual organic plant soil that has been sifted through to remove wood, etc. that are within those bags.

It really depends on the specific bags you may have locally, or the size of the grain (some are sand but are inert with nutrients, others are gravel sized, some are pebble like sized, and then you have the outliers).