I'm plotting and planning an even smaller outdoor system (since a 20 ft pond would take up a quarter of my yard). Maybe I'm looking in all the wrong places (or maybe I just have to pay for the info.) but Denver or even Front Range specific info and tips just aren't out there. I've heard that trout and bluefin are pretty cold hardy and edible. I haven't tried bluefin so can't verify edibility but trout is a bit too mushy for my family's taste. While trout and bluefin may be cold hardy, I don't know what else is and I'm learning that diversity is the key to sustainability. Even beaver seem to know this and have been known to willingly share their own winter food and lodge with muskrat, mice, frogs, and a host of other critters trying to stay warm in the winter. stocking a system with only one kind of animal can be a precarious system to keep balanced and laziness was a significant factor in my conversion to permaculture. I'm not really into frog legs but they're on the list of possibilities. My best thought is to just read up on local natural pond life and replicate it with a few adjustments for edibility.
In the Rocky Mountains beaver ponds are a critical haven of life for surviving the winter. One feature is the deep trenches beaver dig throughout their pond for predator safe passage. These highways also create more thermal layers within the pond. Maybe your whole pond does not have to be deeper but perhaps a 4-5 ft trench could be incorporated into the layout of your otherwise 3 ft pond. Since it would be visible from the air the trench could be in the shape of a fish or peace sign or cross to add some extra kicks and grins to the project. That said make sure you have something living in there that will stir things up in the trench all year round to keep it from getting too anaerobic down there. Alternatively, growbed drains could be plumbed to exit laterally at the bottom of the trench to create some "deep sea" currents. Use a trompe to oxygenate the water all the way to the bottom.
Of course, you could always go for a wider deeper "trench" and incorporate a swimming hole into a portion of the pond. I imagine running of the end of a small dock into a naturally clean pond would feel pretty good in July or August. Just don't stock it with piranhas.
I believe Sepp Holtzer also has his (supposedly tried and true) method of maintaining circulation and survivable winter temps with only contour. It goes something like this: a south facing sloped bottom with the deepest point near the south edge of the water and large submerged rocks somewhere in the middle as thermal batteries. Solar convection moves the water up the sloping bottom which faces the winter sun. the warmed water is pushed across the top of the pond to the south end where it sinks and is again drawn up the south facing slope of the pond bottom. If I correctly attribute this model to Mr. Holtzer, and since he lives in what I'm guessing is a slightly harsher climate than here, and we get more sunshine here than the Sunshine State, it ought to work at least as well as at Sepp's place.
Another way of keeping water temp a tad warmer in winter is to use a greenhouse for shallow winter growbeds. The greenhouse can can be temporary or partially temporary so the beds can be used in summer too in conjunction with your other 2-3 season beds. If aesthetics is a concern, the winter beds can be part of the landscaping but arranged to It should warm the water enough with each cycle to keep the pond going. If the growbeds always retain some of their water, the cold pond water would mix with warmer retained water to hopefully guard against shocking the plants. You could even run it through a small solar collector on the way to the veggies. If anyone has tried some of this I'd love to know what works and what doesn't.
It is important to keep in mind that many fish will hibernate over the winter. There may not be enough fish poop to keep a winter greenhouse healthy.
As far as rainwater is concerned, a fairly eye pleasing "wash" could be constructed (with appropriate liners to prevent soil saturation near foundations) to direct the occassional "flash flood" from the downspouts to the pond via open drainage (rather than the highly suspicious rainwater catchment plumbing). An appropriately sized outlet across the pond would direct nutritious overflow to and across a strategic location before being allowed to freely flow off the property. To size the outlet I'd allow for 3" of rain per hour over the total area of roof and pond. I'm not aware of any storm in Denver that has ever exceeded that (climate change notwithstanding). In my case I have two "orchards" each with 3 fruit trees, a berry hedge, and a perpetual cover crop of chicken fodder that would all benefit from an occasional spring or summer flood. Over flow could be easily directed to flood these areas before exiting the property (if it makes it that far). In Denver this would keep water levels generally up during the summer and provide 1 or 2 partial water changes every year. (Of course, if the system were in place on my property now, this unusually northwestish spring would be yielding a fairly constant stream necessitating a culvert under the front sidewalk and more explicit spillways over the front curb (probably a serendipitous chunk of concrete broken off the edge of the driveway cutout) and the alley-side retaining wall.) At any rate, in this way you are not damming an existing waterway, nor storing it for later use, but directing runoff away from the house and off the property by way of your already full pond (though I'm sure no one would report you if you let a good storm fill your pond initially as well).
To supplement water level with municipal water I prefer (and can afford) more low tech options I will likely rely on a above ground cistern ( barrel or IBC tote) plumbed to manually drain into the pond. I can fill and treat the water in the cistern before releasing it into the pond. Since all the retail pond dechlorinators leave other chemicals in the water that I'd rather not have in my food, I have been successfully treating water for my current system with a 500mg vitamin C tablet dissolved in a 5 gallon bucket of tap water followed by 2-3 days of fresh air and sunshine.
Now that I've waxed eloquent well beyond my experience, I'll wait for more experienced people to weigh in. (The sum of my aquaponics credentials is a 2 year old 40ish gallon indoor system which includes a single very healthy goldfish, 3 houseplants, a first-try bell siphon that has never failed a single time, and a dusty stack of supplies in the garage to build a light bank to allow me to fill my otherwise empty growbed with herbs.)
