Dear Ian, I would recommend ...
Published by Eric Schieblich
Dear Ian,
I would recommend the material of Metamatria PO4-sponge with high capacity and possible to install in nets and can be regenerated if loaded.
secondly I would recommend the floating wetlands from BGS from Germany https://www.bestmann-green-systems.de/produktgruppen/schwimmmatten-und-begruenungssysteme-repofloat-repotex.html
you can use just only the floating system or with plants. Fare as i know patented and the cheapest solution on the market.
I also recommend a design with lines to use the floating wetlands as a guiding for the flow and so that all the biofilm on the rises is always used and you have no dead zones. 40 % of the surface of the pond is minimum recommended for the effect you want to have.
In addition I recommmed to use the OLOID for a movement and flow with the lowest energy demand.
if you need help with the design please let me know.
All the best
Eric Schieblich