Not sure Jules. Weed filters are very effective at stopping water turning green and fish positively thrive with them. They absorb toxic waste and heavy metals and provide oxygen etc etc. Most botanical gardens use them.
Basically it's just mass planting of submerged plants until you achieve a balance and crystal clarity, like in a natural pond full of plant life. Balancing the plants with the water mass and fish is the key.
With koi that is almost impossible, they are big eaters and messy, producing tons of waste matter so you need quite heavy filtration. They also disturb the planting, being bottom feeders so things cant settle. I imagine growing water-lilies with Koi and filtration is possible like Carlo's mum, heavy maintenance is definitely part of it and very robust water-lilies in the calmer end, maybe protected, and very few if any submerged plants?
Natural swimming pools are very fashionable friendly in Europe at the moment and of course environmentally sound, my sister has one. They need to be pretty big if you are going to swim in it, like a mini lake. They avoid domestic fish completely its easier to keep the balance, using tiny wild species that they never feed. In winter they skate on it.
