Yes, with more duckweed and more patience I might be able to run both tanks without filter and still get the ammonia/ammonium and nitrite levels under control. So, I did cheat a bit by adding a filter, but I just wanted to speed up the process a bit.Of course, the irony here is that the name of this site is Aquatic Plant Central, and this forum is devoted to the idea of adding "soil for plants and plants for soil." The whole point of the Walstad method is to lessen the need for ammonia/ammonium conversion since plants will uptake ammonium directly without creating nitrites. It's still possible to accomplish this in the long run. But it would really mean paying a little less attention to bacteria and maybe adding a lot more plants than you presently have.
I did some more water tests yesterday.Now both tanks are like this:
NH4 = <0,05
NO2 = >1
NO3 = 80
pH = 7,5-8,0
I also did a 50% water change and poked the soil with a clean bbq stick. The interesting part was that tank1 (with tiger soil) had a lot of air pockets in the soil. Every time I poked I got air bubbles, in contrast, I got zero air bubbles when I did the same with tank 2 (sowing soil). Both tanks were set up two weeks ago.