I didn't get to the Landen tank WC until Sunday, and I changed 57%. Ammonia is 1.5ppm again today, and nitrates 30ppm -- I dosed NO3 & PO4 after WC, but I hadn't measured anything.
It will do that for about a month and then starts to slow down. I usually dose about 10 ppm PO4 after water changes for the first month, plus the 5-6 ppm it gets from regular dosing per week. This is with high light and hungry stems, slower tanks probably wouldnt need that much but the soil will do its thing regardless. That first month it will zero all that out completely in just a few days. And then say 5 extra for a couple more months. ADA aquasoil does the same thing.
I recall reading about your AS vs. PO4 battle, but I did not realize the same would happen with Landen. Have you also seen massive ammonia spikes with it? (Did you ever buy an ammonia test kit?)
Have you had any bags of Landen that produce dust when you flood them or move stems around?
Based on your experience, I'm going to ramp up my PO4 dosing even further post-WC. I was doing 3ppm post-WC, and I was planning to dose 1.5ppm 3x/week now that I'm moving to weekly WC. I'm going to try 10ppm post-WC and see where it goes.