Treating a pond with Effective Microorganisms® once per month (And, we're usually talking about Activated EM-1®) will keep enough beneficial microbes in the system to keep nutrient levels low enough to improve overall water clarity. Interestingly, EM-1® also has the ability to neutralize salts, such as calcium, and prevent them from crystallizing on surfaces, a great benefit with water fountains using hard water. It will also clean the crystallized salts off the surfaces it comes in contact with.
On average, for a healthy pond or water garden, EM-1® Microbial Inoculant is applied once per month. The rate is 1 part EM-1® for every 10,000 parts water. Allow several weeks to a couple months for the EM-1® to work. Some slime (muck) from the bottom will slowly rise to the surface. Leave it and let nature do its thing as it will disappear shortly.
This chart is a quick reference guide to figure how much EM-1® you may need.