This isn't so much a question as a suggestion, something I remembered while talking to my wife about nitrites and nitrates and why my son's aquarium never has algae, even though it's right next to a window.
Back when I was a teen, I was a huge turtle nut. Actually, I still am, but I have it much more under control now. In my father's backyard, I had a sunken kiddie pool that I used as my turtle pond. On a bi-weekly basis I would empty the pond, clean it and refill it. One of the things that didn't escape my notice, though I did not fully understand the 'why' of it back then, is that the pen area was more green and lush than of any other location of my dad's yard. My dad, being a 'gotta have a pristine green lawn' type of person was always a little baffled how the area around the turtle pen was always much nicer than the rest of the yard, even if I did have dandelions and other 'weeds' growing in the pen. We even ended up with two corn stalks growing up in the pen (the neighborhood used to be a corn field about 6-8 yrs prior).
Ever since then, when I had to clean a turtle tank I would always dump a gallon of the water onto a plant or an area of the yard that I felt needed a boost. Unfortunately, I do not keep turtles right now; two kids, two dogs and two birds and a growing garden are enough to keep me plenty busy. But you can rest assured, that when I have the time, money and space to put one in, I will build a backyard pond for the purpose of keeping turtles. And I will be using that water to keep my plants green and lush.
But in the meantime, I thought I would pass on the idea that if you have a backyard garden pond, you and your plants might benefit from adding a couple of turtles to the mix. Unless you also have aquatic plants in your pond... then you might not be as happy to have little munchers keeping those plants 'in check'.
Oh, and if you do plan on adding some turtles, do not get the cheap Red-Eared Sliders that many pet stores tend to carry. Red-Ears, while super cute, are not indigenous to Florida. Instead, I would suggest finding a breeder or attend a reptile show and look for someone who has River Cooters or, even better, Florida Red-Bellies. The babies are mostly carnivorous and will help to keep insect larvae and minnow populations down (two or even three babies would not be able to decimate a minnow population, they're just not that quick), but they will become more omnivorous as they grow older and will likely snack on plants rather than chase critters.