Algae control in Koi ponds and water gardens is popular topic of discussion among pond and water garden hobbyists the world over. Place a glass of water in direct sunlight and sooner or later, the water will turn green. This green water can be especially annoying to pond owners, many of whom spend thousands of dollars for Japanese Koi fish only to find them difficult to see once an algae bloom takes hold of their pond. Unlike natural bodies of water, most Koi ponds do not have a continuous supply of fresh water. This reality often contributes to “pea soup” green water as excess nutrients build up in outdoor water features, creating the ideal environment for algae to take hold.