- Feeding to much
- Overcrowded Pond
- Not Enough Water Changes
- To Much Sunlight
Overcrowding your pond comes down to how many you have in your pond. Remember koi can grow to 24 inches. They may be cute and tiny now but they will grow fast. I like to say about 100 gallons per koi that gives enough area for them to grow to a decent size to allow you to upgrade.
Water changes can be difficult to do in a pond that is why you want to use a pond vacuum. This makes it easy to remove the excess nutrients in the water. Making less food for the algae to eat off of. Pond Vacuums come in many different sizes and are simple to use. I'd like to think of it as a pool vacuum pretty much but different so that you don't scratch or rip the liners of your pond. Also check out the Matala Muck Vac II, Pond Vacuums Muck Vac,Pondovac 4,Poolmaster 28008 Black Magic Spa/Pool/Pond Jet Vacuum
Is to much sunlight entering your pond. This is what algae feeds off of also and when you have a over abundance of sunlight and nutrients you have a instance algae bloom or green water in your pond. You can use a blue tint liquid that is made for ponds that helps reflect the sunlight from the pond. Also you can build a awning over the sunny side of the pond. Even stopping the early morning sunlight will reduce your algae bloom tremendously
If you are still having problems with your pond having green water. Please leave a comment and I will answer it as quick as I can.