Substrate for natural pool

Hi. So I'm building a swimming pond. David Pagan Butler mentions that a substrate of subsoil and sand mixed in equal parts is good enough. But, my subsoil is clay. So if I mix that with sand I'd get cob right? And that would be too hard for the water to pass through to the pipes and the plants to grow. Should I just use crushed rock gravel (20mm) and crushed sand mixed 50/50 instead?

Is the substrate for the plants to grow in, rather than a pond liner? I'm not familiar with the design of natural swimming ponds. So the water is sucked through the substrate and into the bubbler and back to the pond?
On his website he says that some subsoils maybe too rich in nutrients, but doesn't say anything about permeability. I'm inclined to agree with you that clay may not be the best to use. I suspect the plants would be OK, but the water would not be able to flow through it so easily. If you have a source of sand/gravel that seems sensible to use instead. Can you use the clay for landscaping instead?

Yes the substrate is for the plants to grow in. And the water has to pass through the roots of the plants and out of the bubbler to get filtered. The heavy clay sand mixture would be too heavy for the water to pass I think. So I will go for a 50/50 sand gravel mix. I think the plants would be OK.