When I was building my pond about 12 years ago there was some talk on rec.ponds about "veggie filters". So I built my pond with that in mind and used a 150 gallon stock tank as a combo settling chamber/veggie filter. At the time I also had a lot of potted plants in the pond that also pulled out nutrients. The results were mixed with fairly clear water and an occasional algae bloom in the early spring before the lily pads gave me adequate coverage.
Two years ago I added two mature red eared sliders to the equation and they all but destroyed my lilies. I had green water the entire year. I then gave the turtles away and I was next plagued with two raccoons that ate my water hyacinths, dumped several potted plants into the pond and chewed a hole in the liner beneath the bridge. I had green water for the 2nd year in a row. I never test my water....but the fish have never complained....
This last winter I converted my stock tank to a Skippy biofilter using Savio Springflo as my biofilter media. The results have been fantastic with beautiful clear water.