I would suggest 3" total, especially if you plan to heavy deep rooters like crypts and swords. This will ensuer a lot of bedding for the roots to spread in and less liklihood that some of the roots will make their way back to the surface as could happen with much less substrate. IME, 3" is also a good amount to keep your plants well anchored and rooted, especially if you have fish that decide to run rough shot over your plants.
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