Every koi pond must have mechanical filtration, biological filtration, irradiation and filtration by plants. In nature every Koi has about 100,000 gallons or more. They do not face shortage of oxygen, pH and temperature swings, ammonia, nitrite and nitrate spikes in rivers. Moreover, minerals are replenished with every rain that brings new water. In ponds, they face all these problems if the pond is not well designed or well filtered. So, it is essential to understand why we must filter pond water in many different ways.

Mechanical filters
They remove solid particles such as koi feces, scales and teeth, plant materials, dead nitrififying bacteria, and algae etc. Most of these are heavier than water and therefore accumulate on the floor. So, bottom drains must clear them from the pond. To be able to do so, the bottom drains must be installed at the lowest level on the floor. It is a good idea to have bowl shaped bottom that is sloped down towards bottom drains.

It is important to install the pump after mechanical filter in the bottom drain circuit. Otherwise, the pump can grind the solids into fine particles that are too small to be filtered. Water flowing via gravity prevents accidental pond drains in case of plumbing failure. To be able to pull water by gravity, water level in the mechanical filters must be at the same height as the water level in the pond.

Biological filters
They process dissolved ammonia, sugars, proteins, nitrites etc. Cloudy pond water is a result of any one or more of these. Koi, and other living organisms (frogs, snails, worms) produce most of these but they can also be released by decaying algae and plant materials.

Biological filters must be installed after the mechanical filters or else the media can get coated or jammed with solids. Water from the mechanical filter enters the biological filter via gravity. This means water level in the biological filter must match the level in the mechanical filter and the pond (when pump is at rest). When you start the pump however, water level may go down by a couple of inches. If it is too low, then the pump may cavitate if pond loses an inch or two of water due to evaporation. To avoid cavitation, the outlets in the biological filter (that supply water to the pump) must be at least 6 inches lower than the water level in the biological filter (when pump is at rest).

UV sterilizers
UV sterilizers do NOT actually sterilize pond water because water moves too fast through them. However, they kill suspended bacteria and algae. They do NOT kill algae growing on the pond walls or string algae. They kill the algae that pass through the UV tube and prevent green water problems.

Bacteria present in the biological filters convert ammonia to nitrate. Nitrate is toxic to koi and there is no way to remove it easily. One can install a de-gassing equipment that forces nitrogen out of water. But easier and natural way of removal is through plants. Plants not only absorb nitrates but also phosphates and other pollutants present in pond water.

Like koi, plants and algae take up oxygen present in water for respiration and release carbon dioxide into water at all times. However, when there is sunlight, plants and algae also use carbon dioxide for photosynthesis and release oxygen into water. So, there is no net gain or loss in oxygen or carbon dioxide when there is daylight. But there is no photosynthesis at night or on cloudy days. In such situations, the carbon dioxide released into water by plants and algae can bring down the pH of water and the oxygen plants and algae take up can reduce the amount available to koi. So, do NOT overcrowd the pond with plants. Discourage algae growth by planting shade trees or installing shade sails. Keeping the alkalinity around 120- 200ppm helps in buffering the pH swings created by algae and plants.

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