There are two reasons you can cycle so quickly in a Walstad type set up.
First, when you add a bunch of plants into the tank you have a massive supply of the bacteria that you need to handle the ammonia, nitrites and nitrates (it’s all over the surface area of the plants). The plants also use these as well. I often temporarily put Najas grass into a tank to remove spikes in ammonia while the other plants establish themselves. Floating plants like duckweed or tank grown water lettuce can also help when a tank is first set up.
Second, Walstad style tanks often use low stocking levels for fish. Because of the low stocking levels and high number of plants, you can add the small number of fish right away.
When I am setting up a tank, I do not cycle, except I might wait a day for the water temperature to come up to the temperature that I want. But, you need to remember I am happy with 3, 4” fish in a 90 gallon tank.
CO2 is not part of a low-tech type set up but can really help plant growth. Just be careful as there is a thing as too much CO2 in a tank.
