hmm. Interesting hoppy! I simply sprinkled my clay into the tank so it slowly sunk into the substrate. Of course this tank is dirted already, but after about 2 weeks the clay began to give the ludwigia arcuata of which i sprinkled it around, an orange - red color tone to it. Of course this could be something else but i haven't changed a darn thing for months. Reason i added the clay is cause i did a little research (mostly on TPT) and people also had the same experience..I think the clay is there mostly to hold the nutrient ions for the roots to get to them easier. I don't think the iron in clay is accessible to the roots until bacterial activity make it accessible. But, I'm far from an expert in soils. Chelated iron is immediately usable by the plants.
Also i must point out i'm not an expert to soils either. I merely test, and observe!
here is the thread i read, i focused on your post and the following post under you.
p.s. (thought i might add) im not using pottery clay, i just got some red clay from the yard, dried it for a week in the greenhouse, and crushed it up to Very fine (under 1mm>) dust like sizes.