Richie,
From what I have learned, a NSP is not a pond. About the only thing in common is the water and plants. Gallop said it the best, it�s �an aquarium big enough for humans to swim in�. (But not for fish)
To make it work the PH has to be low, 5.5 to 6.5 maximum. For a pond with healthy fish the minimum PH is 6.5 IIRC. Plus fish of any size will add too much nutrients (poop) which would require a larger filtration area. I may add minnows if mosquitoes become a problem but will have to limit their numbers. The literature says mosquitoes are not a problem because the water is always moving.
The filtration area (6� to 30� deep) needs to be minimum equal to or 1.5x the area of the pool (36� or deeper). You also have to have water constantly circulating through the gravel in the filtration area. I don�t remember off hand the water changes but it�s more than you want to do for any decent size pond. My design has the existing pool of 600sf and a new filtration area of 1,100sf. .04 acres total.
When I first got my pond last year I too thought it would be great to turn it into a NSP until I started reading Pond Boss Forum and about NSPs. I calculated the high 5 figures if I was lucky to turn my 2 acre pond into a 3/4 acre NPS. They are different creatures.
Lastly, it will be a natural "water garden" not a natural "swimming pool". There are too many building codes for swimming pools. I can�t help it if, due to an excess of beer, I fall out of the hot tub into the water garden every once in awhile.
