I do nothing to my pond water. No filter. Just still water. The plants (water lilies) seem to keep it clean. I occasionally get an algae bloom, but it will clear up on it's own after a week or so. If I get string algae, I just use my fingers and twist and pull it right out. But the water usually stays really clear and the fish seem to be quite happy!
My biggest issue is the tadpoles. I have to scoop hundreds of eggs out of the water constantly. Seems the frogs like the crystal clear water for mating and laying eggs. Ugh!
Water gardening is the easiest gardening I do! Takes the least amount of effort except once a year when I clean the small ponds out and repot and fertilize the water lilies and marginals. I have to put the fish in a bucket temporarily when I do that, too. Tadpoles get dumped. I don't need another single frog here! They are Cuban Tree frogs and very invasive critters. Noisy too! But everything else is a real delight and so easy to maintain. The best part is that I don't have water any of the plants every 2 days. Since they grow in water, I just have to top the pond off once a week from evaporation. And I use mosquito dunks to keep the mosquitoes from multiplying in my water containers. The fish help control the mosquito larvae too.
I love how nature balances everything eventually!
Paul - Those Lotus pots were cheapie containers from Wally World. I got them because there were no pre-drilled holes in the bottom and they had that asian symbol on the side of the containers. I have two of them. One on each side of the arbor.