I followed up the rhubarb pesticide question with the website where I found the recipe. This was the emailed response:
I would imagine that a high enough concentration of rhubarb leaf pesticide in a small volume of water could harm fish but in a garden-pond scenario with a small dose that there would be little risk. However, by avoiding spraying the pesticide into the water itself you will avoid any risk to the fish.
ecologica di Jonathan Radford
So I guess that used with care, it should be OK.
I have received the brochure and a sample from the Keder greenhouse people, CLM. They look like everything I could want, but the prices are high enough to contract the sphincter muscles! I shall have to downsize my expectations – or possibly live in one! But they look fabulous. Exactly what I need.