In the Walstad approach, the eventual depletion of the soil nutrients is offset by the fertilizer that well-fed fish produce.

I have one soil-based tank that has been set up for eight months and needs no dosing, and another 10 month old tank that has required dosing from the third month, although the amount needed is decreasing as the fish population grows and the plant growth slows down.

All natural aquarium environments are different and perhaps more interesting than those with more controlled environments. Or perhaps not, depending on one's objectives.

Bill