I have read numerous times that using the walstad method can result in a potasium (K) deficiancy after the tank has been set up for 6 or more months. The reason for this is that there is little potasium in fish food, the potasium in the soil is almost the only source of this macro nutrient. Once the plants have used the limited amount of K present in the soil they become defiant, pin holes develop in the leaves and growth is stunted. The potasium is removed from the tank through the prunings tacken from the tank. From what I have learnt about permaculture I know this is not sustainable. I want to try find a solution to this. I haven't come up with any organic solutions so I thought I would try this as I had a big bag of this from when I used chemical fertilisers. I realy don't like using chemicals :(
I thought I should mention now that I haven't had a K defiancy in any if my tanks yet (nearly a year) and I hope I don't develop one. I might add some sea weed to the filter to decompose, it is supposed to be high in K.
There are three options I can think of, or two I suppose depending on how you look at it. You could a) strip the tank down again b) fertilise using a water soluble fertiliser or c) add fertiliser rich in K to the substrate, I would prefer something organic but I can't think of anything off the top of my head.
A tip I recently learnt is this, you can add potasium sulphate to the soil before you add it to the tank. I decided to give this a try, I added 1 table spoon a small ice cream bucket full of soil. I'll see how it goes. I'm worried the sulphur may be turned into hydrogen sulphide from the anearobic bacteria in the soil, only time will tell. I can't remember where I read this so I just added what I thought would be a good amount, the tank has been set up a few days and the fish and plants are fine so far. I'll keep you updated on the progress.