Avoiding and Removing Algae

no sun, no algae.

Chemicals, radiation and filtering (see your pool guy).

Is your question with regard to a swimming pool or a pond? If it's the latter, then check out this link.

For a swimming pool, it's by dosing in enough chlorine to get 1ppm free Cl₂ in the water. For a pond, growing plants such as water lilies that will shade the surface or hornwort, Ceratophyllum spp, that exhibit alleopathic behavior i.e. they secrete enzymes that suppress the growth of algae. Adding a couple of bales of barley hay also helps as they release similar enzymes.

How about fish that eat it.

If you are reffering to a pond, they make an environmentally friendly copper solution that shows fairly good results. Another option is aeration. Anaerobic bacteria will reduce phosphate levels in the pond which is the limiting constituent for algae growth. If you have some sort of pump system hooked up to the body of water, I would recommend injecting a phosphate removal additive. These are usually inert and poses no health risk to the fish.

If the water body you're referring to is a natural pond or lagoon, where most likely other aquatic creatures thrive, you can't avoid the formation of algae.

Chemicals may control the growth of algae, but it won't be helpful for the other aquatic animals. It's a natural phenomenon, so there's no use resisting it. If you really want to do something about, try scraping those areas regularly, and installing some shades. The usefulness of shades, of course, are inversely proportional to the size of the water body. The bigger the water body, the less useful the shade, unless of course, if you can afford to cover the whole area.