I use 19 mm polypipe with a joiner but the hoops I make are quite large. Planted with Pistia stratiotes (Water Lettuce) they're good biofilters. But any water plant suited to the climate would work well. There's lots of possibilities.

How did you join up the ends? I need that for my hyacinths. I will also have to add a netting on the bottom so the koi don't eat all the roots. btw, where do you buy such cheap water plants? locally? online?

Tropicbreezent.....Is water lettuce legal in your area?

Catherinet, Water Lettuce is a native here so no problems. We have a moth that keeps it under control. In fact, this year the floating mass in my photo above was virtually wiped out by the moth larvae. The problem with 'monoculture' - ideal conditions for multiplication of pests. Other years I never had the problem. With the explosion of the moth population they also got into my Victoria Lily seedlings and decimated those. Might have to pick another water plant for use as a biofilter.