UV filters are 100% effective. It's not well understood why water sometimes goes green and some times doesn't. It is known that plants give off chemicals that inhibit growth of other plants. For ponds G.P. Fitzgerald in the Journal of Phycology, 1969, showed Pithophora (a common species of string algae) creates a chemical in the water that kills green water algae. In my own experiments I've seen this too and also that green water algae can kill string algae. It's a plant war and the winner is never certain. Norm Meck published an experiment that showed water from a clear pond was toxic to green water algae, kill it on contact, although he guessed the chemical came from bacteria. There are some aquarists doing experiments to see if other submerged plants do this as well. Likely there are since this type of chemical warfare is very common in the plant world. It's also been demonstrated many times that green ponds almost always test 0 for needed nutrients while clear ponds test high for nutrients. While nutrients can certainly make green ponds even greener nutrients are not directly related to clear water. If you chase the myth of trying to lower nutrients to get clear water you could just be spinning your wheels. Maybe even hurting the plants that may had been producing the algae killing chemicals. Emptying water and replacing would only be a short term fix. Normally it produces an even greener pond within a week or two. There are copper based algaecides. The trick is to get the dose right as it can kill fish and higher plants. Adding copper metal isn't the same thing. Metal releases so few ions that it isn't effective. Copper based algaecides are effective. These are complex living systems. Impossible to guess what tomorrow will bring. But if you stick with science and stay away from myths you increase your chances imo of getting the results you're after.

I wouldn't add goldfish, or any fish. I tried a small pond that was in the ground and the water got too hot for mosquitofish which can take warmer water than goldfish. Being completely above ground even in the shade I'd expect the water temp to be way too high. With the dark color and 50% in the sun the water might be too hot even for plants, or maybe some plants. I've been growing Canna and some kind of Umbrella Plant like Cyperus alternifolius and a nutsedge (purple I think) in soil in containers with no drainage, so standing water but normally below the soil level. That mimics how I keep marginal plants in a pond which grow best not submerged. The sun, heat and humidity are hard on these. These are all pretty much in the ground, but the further up out of the ground the least well they do. I've repotted plants that have been in the sun and the soil is much too hot for me to touch. I've been experimenting with different sun exposure. Canna barely survive full sun. Morning sun seems to be best, til 10 am. Enough sun to bloom, not too much to stunt them. So I don't really have any tips for what you're wanting. If you want a water feature I suggest you just do that. Then add plants for the cooler months, or if some make it thru the summer then great (let me know what). Maybe something like a pondless feature. 5 gal bucket with holes goes contains a small pump, kept up off the bottom by a brick. Then fill with rocks/stone around the bucket. At the very top you can add a more expensive rock for looks. Add water so it stays below the rocks. The fountain hits the rocks and sinks back down to the pump. Plants can be placed in the gravel/rocks either bare root or the whole pot with soil. But you have to make sure the water level stays up to at least say 6" from the top. A fountain might be hard on most plants which don't like to be covered in water. And you could get lime deposits on leaves. Because there's no standing water you don't have to deal with string algae or mosquito larva. I've had container ponds other places, NY, FL, CA, and none of that applies in Phoenix. I used to read gardening books and web sites and watch gardening TV shows, but don't any more...just doesn't apply to us.

I always thought it would be really entertaining to stock the retention pond behind the house with Asian Carp. Those are the ones you may have seen on the news that jump out of the water and fly into passing boats. They'd probably grow legs and walk out of the pond and start eating people. Here is a link that might be useful: Asian Carp

The best way to grow waterlilies is from tubers (tropicals) or rhizomes (hardies). I have grown waterlilies from seed....and you can get a tropical to bloom in the first season but a hardy will take two years or longer. Most people who grow them, grow them from divisions, since almost all are hybrids and won't bloom true from seeds. It can be interesting for a just for fun project. Tropical seeds are small...similar to poppy seeds. The way to plant them is to use finely sifted garden soil. Plant them and then cover the soil with some fine sand...like what is used for sand boxes...play sand. Lower carefully in water...like in an aquarium where there are no fish. Grow under lights....germinate usually in 10 to 14 days. However, they can be plagued by algae and fungus. so I wouldn't bother with it if that is your aim to have plants for a watergarden....I'd stick with buying or trading for plants. Hope this helps. I have grown waterlilies for almost 50 years.