Alright I think an update to the whole pool project is in order. Due to a lot of other project having priority, the pool thing became a quick experiment that I was running on the side. Although I would have loved to build a gazebo on top and all that, I simply did not have the time. SO, instead I started growing duckweed for the chickens, and they loved it! The pool had been left alone for a year prior to me adding the duckweed, so for the first season production was great, I pulled 2 to 3 kilos (4,5 to 6,5 pounds) of duckweed out every day during the summer for the chickens to feed on. the year after that production went down, not a lot, but enough for me to notice, so that year I regularly peed in the pool to keep the nitrogen levels up. I also considered putting carp in the pool, but the duckweed had formed a dense green mat so no light was penetrating and I think the oxygen levels would be pretty low... So last season (2014) I came up with a plan after doing research on the aquatic plant, Azolla. This has turned out to be a spectacular success that I can only recommend.
In the spring I threw in a small bag of Azolla, about half a kilo, just as the duckweed was coming out of dormancy. The duckweed seemed to take over at first, but then the Azolla got going and quickly started to dominate the pool, the chickens are happy to feed on both, although the duckweed is better for them nutrition-wise, a little sorting while harvesting is required to keep the duckweed from going extinct. The awesome thing about this system is that I now get several things from the pool. The Azolla fixes nitrogen in the pool, feeding the duckweed, but it does this to such an extend that the water becomes very nitrogen rich, making the water perfect for watering other plants, even the house pots. Every day during the summer (about 3 months here in Denmark) I can pull out 5(!) kilos of Azolla-duckweed mix for the chickens, OR for a boost for the compost. I have even been using it for mulch in my raised beds as the chickens free range during the summer and dont need much extra feed.
So this year I am combining this system with a small solar hydroponics setup, with 3 rain gutters, each 3 meters long with watercress in them, and then have the water from the pool run through before going back in the pool. It might be too big a nutrient drain on the system without adding fish, but I will know more come autumn.
Please note, if you want to try this, Azolla is hyper-invasive and it spreads via spores like other ferns, if it reaches a waterway, it will spread like crazy and potentially poison lakes, rivers or creeks with nitrogen, which then results in algae blooms, oxygen deprivation and so on. Frost does kill it, but all it takes is one little piece surviving somewhere and come spring, it will explode again. In fact, this plant may have changed the climate at one point in the past as described in the Azolla Event http://en.wikipedia.org/wiki/Azolla_event
here are some pictures of the pool at the end of this years season, the duckweed has gone dormant and the azolla is struggling with the cold.