Good evening, KATIE – I just wanted to let you know that we had not forgotten about you and your question. Our experts are just super busy this time of year. Please know you are in our queue and we will get you very soon. Best wishes.

Katie,

Thank you for reaching out to MSU Extension Ask Extension with your garden pond question. Small garden ponds can be quite tricky to manage. I own a small garden pond with goldfish that I keep year around. It takes daily attention to feed and monitor fish health.
There are a few things that I do to maintain an algae free garden pond.

• I have a secondary 1.5 cu ft filter box after the pump and filter and before the first waterfall. (You can make your own or buy a ready made one) I made my own using an old urn shaped flowerpot. It has been painted several times. Currently it is sporting a nice forest green. See photo
• I then have an upper and lower pond structure that creates a small waterfall. The upper structure is filled with plants that help to take up excess nutrients.
• I have selected potted lily pads and floating Coontail in both the upper and lower ponds for shade to keep the water cooler and to help shade out the algae.
• We clean pump filter monthly and the big secondary filter annually.
• I vacuum decomposing organic materials from the bottom twice a year with a small pool cleaner connected to my hose.
• We skim several times a week and daily when blossoms and leaves are falling into it.
• We feed our fish once a day with a low/no algae pellet (over feeding fish just feeds the algae) My15 fish get 1 tablespoon of pellets once a day
• String algae is normal and grows on the fountain, walls, outfall, and waterfall. I do clean it when it gets out of control, but the fish also feed on it. It does not turn the water green. It will coat everything in the pond though. The protective shelter for the fish to hide from predators and submersed flowerpots are completely coated and are now all the same color as the rest of the pond.
• we do a corrective clean as noted below every three years or so as needed usually in late April. 

Now, if your pond is “pea soup” green that is an entirely different kind of algae. This one grows from too many nutrients in the water either from surface water runoff or too much fish food and stagnation. There is probably a lot of sediment in the bottom of the pond as well.

My recommendation to correct is to

• add some algaecide per package directions
• the next day
  • pump out 25 to 50% of your water (it is okay to use the water in your garden)
  • pull up all your potted plants, if you have any, and set in a tote in the shade with some water in the bottom so they don't dry out. 
  • scoop out duckweed with the plants and put it back in later with other pond plants
  • scoop debris from the bottom, skim the top with a handheld pool net skimmer
  • clean all pumps and filters thoroughly
  • reset pumps, filters, fountains and plants
  • refill the pond
  • it will look really bad at this point, but it will settle again in a few hours after you reset the pond
  • Keep your filter pump running all the time.  Shutting it off creates stagnant water that will turn green and grow algae and begin to smell bad very quickly. If you want to shut off a fountain put that on a seperate smaller pump that you can unplug or turn off.
  • optionally you can add some pond dye if you do not have plants to help shade the water reducing algae growith.
  • Note: If you have fish you must manage your pond more like an aquarium and less like a natural pond.
    Add aquarium salt and fish treatments according to package directions if you have fish. 

At this point you can trust it will stay clean and fairly algae free with the above-mentioned maintenance schedule.

Happy gardening!