• Like the water in your aquarium, your pond water must be regularly tested and maintained.

    Your pond should be tested for most of the same parameters that apply to the indoor tank—ammonia, nitrite, nitrate, pH and hardness (GH/KH). Additionally, pond water needs to be tested for oxygen content and phosphate levels.

    Although the water maintenance concepts are the same between tank and pond, the environmental differences warrant purchasing water test kits made specifically for pond use. Pond test kits are calibrated to report findings on a different scale than aquarium test kits since, for example, ammonia and nitrite are far more toxic in a closed aquarium system containing a small volume of water.

    • Ammonia, nitrites and nitrates: The most common causes of high and unsafe readings are:

      • Insufficient number of live plants
      • Malfunctioning filtration system
      • Profusion of dirt or decaying material
      • Too many fish

      A pond is at its best when the ratio between the number and size of fish to the volume of water is balanced. A small population, well-functioning filtration system and balanced fish-plant mix will help in maintaining water quality and stability in a clean pond. However, goldfish and koi can become quite large and are messy eaters that produce a lot of waste. There are circumstances where fish like these will outgrow the size of the pond and exceed a comfortable fish-to-water volume ratio. If you suspect that your pond is becoming overcrowded, test the water often. The pond will let you know how it’s handling the stress.

      Check to be sure your filter is functioning properly. There may be too much dirt or decaying organic matter trashing up your system; dead leaves, algae, insect larvae, fish and animal waste can add up to a pretty big problem. Physically remove as many of these contaminants as you can from the pond. A partial water change is a good idea for temporarily lowering ammonia and nitrite levels, but the only real solution is to eliminate the source of these substances.

      If your pond contains the right mix of live plants and fish, nitrates should never rise to an appreciable level. There should be plenty of greenery in the pond to consume most, if not all, of the nitrates. Failing that, regular, partial water changes are the only solution.

    • pH: Like an aquarium, pH levels should test at 7.0 or slightly lower. The presence of plants, potting soil and dirt will do its part to lower pH over time. The fish themselves help to maintain safe levels, as pH is lowered through byproducts of their respiration. The evaporative process and the replenishment of pond water from rain also will help maintain a natural, healthy pH.
    • Hardness: Low water hardness measurements can result in insufficient levels of calcium and unsafe swings in pH. A common cause of low hardness is the buildup of organic matter on the bottom of the pond. This decomposition produces nitric acids which will affect the pH and KH. High water hardness can result in scaling and can plug pumps, filters and pipelines. Live plants and buffering aids all can be used to modify water hardness.
    • Oxygen: If you suspect that the pond is getting overcrowded, or if water temperature is very hot, you’ll want to keep an eye on the oxygen level. Though fish don’t respire the same way we do, they do need plenty of oxygen, and warm water holds less than cold. If the pond is overcrowded, oxygen may drop to an unhealthy level; if you see fish gasping at the surface, lack of dissolved oxygen may be an issue.

      As with most pond problems, there is a pretty easy solution: you can increase the oxygen level with partial water changes, add surface agitation to the pond with a waterfall or fountain, or add an underwater aeration device. Any sort of constant movement in the water should have things back to normal in no time.

    When you’re first getting your pond up and running, you’ll want to test your water frequently. More established ponds can go a week or so between tests. Unless conditions get extreme on either end of the thermometer, you don’t need to worry about testing for temperature.

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    The lakes of Japan where koi are raised are fed each spring by natural melt water from the mountains.