DO NOT DRAIN YOUR POND IF AT ALL POSSIBLE. If you're trying to keep the algae down, there are two methods of attack. Aquatic plants utilize ammonium and nitrate (by-products of fish waste) and require light for photosynthesis. Install a properly sized UV filter (runs in line with a water pump), this will burn algae spores, large parasites etc.
The other important thing you should do is put in some plants. The plants compete directly with the algae for food and light. Plants also improve water quality for fish, and provide needed hiding spots. A general rule of thumb with ponds is cover 1/3 of the total surface area with plants. If you have any questions feel free to e-mail me as I've got an education in aquaculture, was formally a fish wholesaler, and have designed and installed large filtration systems for both retail stores and pond owners for the last 10 years or so. (I do admit though that my knowledge on pond plants is weak.) but I'll help out as best I can.