Plants, plants and more plants. That was always the suggestion from my local pond store. Also, sometimes one needs to just wait it out - the pond needs to be balanced to be healthy. When I had a small open pond, it went through a couple unsightly algae blooms, but eventually sorted itself out. I didn't use any chemicals as this was the main water source for my dog and cats. If you live in the vicinity of dragonflies, I've read that they are a good sign of pond health. I chose that badge for my pond interest rather than the water lily. I now enjoy a pondless water feature, and still have dragonflies so I guess the water below the rocks must be in fairly good balance. When we first converted to pondless, I had chronic string algae on the rock columns (I just scrubbed it off), but now in its second year, not a problem.
Another thing I would occasionally do with my open pond was to add buckets of 'real' pond water from our creek-fed farm pond. I figured it had all sorts of growing things in it and the goldfish always went nuts when I poured in a bucket. Good luck!
