We have a cement-lined stone pond about 6 feet long by 4 feet wide by 2.5 feet deep (1.8 x 1.2 x 0.8 meters). We use well water with a pH of about 7.4. The pond has a 4-foot high waterfall that is driven by a submersible pump. The pond contains six ordinary goldfish, each about 4 to 6 inches (10 to 15 centimeters) in length. Our problem is that the water becomes murky and overloaded with algae. We drain the pond and scrub out the algae, but in almost no time it becomes murky again. How can we control the algae?
The murkiness in your water is suspended algae. Depending on the mix of algae, the nutrient levels, the amount of sunlight that hits the water and the time of year, the water may look deep green, greenish-brown or brownish-black.
The problem of controlling suspended algae in fish ponds is probably raised more often than any other question. I am sure it will not surprise you to learn that a comprehensive answer would take up many pages. Let me offer some straightforward solutions.
Algae are plants, and they require three things to grow: water, light and nutrients (phosphorus, in particular). Because the water is a given, let us consider the other two factors.
Your water evidently has a high nutrient content. The best way to starve out the suspended algae is to put other plants in the water that will compete for the nutrients. Water hyacinth, iris, parrots feather and any submerged oxygenating plants are all good choices.
Most importantly, I would stop scrubbing down the pond walls. The algae growing on the sides is not the same algae that is making the water murky. If you let the algae on the walls grow, it will help starve out the suspended algae. It is also a good food source for your goldfish.
Another way to cut down the nutrients is to install a biological filter. I estimate your pond holds about 360 to 400 gallons (about 1500 liters). There are many options for installing a small biofilter, but an easy solution is to use one of the foam-sleeved Danner (Supreme) Universal Pond filters. This just attaches to the inlet side of your submersible pump. It will remove suspended debris from the water and become a biological filter after a month or two. All you need to do is gently rinse out the foam sleeve every week.
You also might consider cutting down the sunlight reaching the pond. Many people in your area of the country install a sunscreen or awnings. Others use wood lathing or some type of screening material.
Alternatively, if your pond receives six or more hours of sunlight daily you could grow water lilies. I would try to cover about 70 percent of the pond’s surface with them.