How can you kill algae in a fish pond?

Start of by scooping out as much as possible hen take care that you don't overfeed the fish as this will keep it going. as for actually killing it without killing the fish I don't know. If the fish pond isn't too big try replacing a large proportion of the water.

Get some fish that like to eat that stuff.

I have a 6-foot diameter 3-foot deep pond with a bog & waterfall circulation stream (1/6 hp pump, 15 gallons per minute). There are 4 koi (with babies), 3 goldfish, snails, lillies and watercress. The moss grows too vigorously in the summer, so I just brush and skim it out about once every couple of weeks. Some things that help reduce moss growth: shade or covering (lillies work to do this), sand, brushing and skimming, optimal amount of feed for fish (no leftovers).

2. If you want to actually kill it immediately, you will need to use an algaecide. GreenClean is very good fish-safe algaecide that does not use copper based ingredients.

As far as long term removal and control of algae, you will need to control nutrients in the pond meaning excess fish food, fish waste, leaves and grass clippings and even fertilizer run-off. This can be done by a proper filtration system that will filter out these contaminants. Also a good beneficial pond bacteria added regularly will also help to degrade these wastes quickly.

Another step would be to aerate/ circulate the water in the pond. Algae hates oxygen so by improving oxygen levels wither with a fountain, waterfall or even diffused aerators, you will also help to control algae.

A third step would be to limit sunlight penetration. Two common ways to achieve this is to add blue dye to the water to help filter sunlight. Pon dye is typically non staining and will not harm animals, fish or plants once mixed with water. Pond plants can also be added to shade the water. They also offering a filtering effect to as they will help pull nitrates from the water that algae will use as food.

Another option available (although more costly) would be to zap it with an ultrasonic waves using a device that floats in the pond. There is some good info on this technology at regarding their Ultrasonic Algae Control. It would be worth taking a look.