I have a small pond, 36' x 40' x 10'
This year it is bright green with algae.
I have 20 ducks and 13 turtles living in it.
Last year there were Blue Gills and frogs.
I am worried about stagnation.
This year I started pulling water from the bank and pumping it back in to help move the water.
Do I need to treat the water? Do I have to many animals for the size?
Can I put in bottom feeder fish to help?
Mecosta County Michigan pond and water gardens
As long as you have a positive plant ID and know you have algae (very important) you can treat the pond. A healthy pond with fish in it will have 30% cover of native aquatic plant species that provide food and shelter for the life in the pond.
Is this filamentous algae, a structured alga (E.G. Chara or Nitella) or a floating alga (makes the water soupy looking)? That will make a difference. No, triploid grass carp will not do a good job on filamentous algae, they really don't like to eat it. You may also need a permit to stock them in your pond, and you need to be careful that they are sterile and can't escape, as they are an invasive species. Catfish and other bottom feeders will most likely cause muddy water problems. If teh sunfish are breeding, you may want to consider stocking a native bass to keep them in balance. we have information on fisheries management at http://extension.psu.edu/natural-resources/water/ponds/pond-management/fisheries.
Most algae and plant problems are really nutrient problems. I suspect the droppings from the 20 ducks are part of the problem. This will cause a huge bacteria and nutrient load in a pond. You didn't mention if they are domestic ducks or wild ducks that moved in. If wild, discourage them. Establish a no-mow buffer around the pond to keep fertilizer and grass clippings out of the pond. Those are a problem. You can mow winding paths for pond access. Add appropriate shoreline plants and flowering perennials to attract birds and butterflies. Your local master gardeners may be able to help you with plant suggestions. Specify that you want to use ONLY native plants for pond health. if you don't have other types of aquatic plants (floating leaf, submerged) to use up some of the nutrients, they will all go to making algae. Too much plant growth of any type will eventually die, decay and lower the oxygen in the water enough to possibly kill your fish. http://extension.psu.edu/natural-resources/water/ponds has lots of good information that can help you. http://extension.psu.edu/natural-resources/water/ponds/aquatic-plants has great videos to help you understand your pond.
Look at the last pages of Management of Aquatic Plants for herbicide suggestions, if you want to go that route. It is a temporary fix, but may work for now. Based on a good plant ID, read about the herbicide you want to use and its toxicity to firh and wildlife. consider a bubbler-type aeration system to help break down the nutrients, there are bacterial additives to help with nutrient breakdown, and peroxide-type herbicides that can also help clear the algae without using potentially toxic copper compounds (used to control most algae).
Sorry if this is a bit generic, but I need more details to be more specific.