by steve hopkins

Koi and goldfish keepers have a love and hate dilemma when it comes to green water. By "green water" we mean pond water with an abundance of tiny single-cell algae. It is not the hairy string algae attached to the pond sides. There are many species of single-cell algae which can contribute to green water, but almost all of them have the same effect.

These single-cell algae are very easy to cultivate in outdoor ponds. Some say it’s much too easy. Just add water, sunlight and fish food. When conditions are right, water can go from clear to green almost overnight.

Many koi and goldfish keepers invest considerable time, effort and money to rid a pond of green water and keep it clear. What is the use of having beautiful fish if you cannot see them? For others, maintaining just the right hue of green has become an art and integral part of their fish husbandry skills.


Single cell algae are pretty good at capturing ammonia and other sources of nitrogen which they use as a fertilizer. Since the single cell algae are floating around in the pond, they have an opportunity to capture any available ammonia before it gets to the biological filter. The nitrifying bacteria that capture ammonia in the biological filter are mostly attached forms and not many of them are floating around in the pond. Thus, green water may, or may not, be an indication that the biological filter is undersized. Even if there are plenty of nitrifying bacteria in the biological filter, the single cell may out-compete them for the available ammonia.

For a koi display pond, a ultraviolet light (UV) sterilizer or clarifier is often used to rid the pond of green water.

The UV unit is usually installed so that all or a portion of the water being passing through the biological or mechanical filter, also passes through the UV. Some say it is better to put the UV unit after the other filters so that the water passing through the UV is as clean as possible. If the biological and/or mechanical filters are not pressurized and water flows by gravity back to the pond, then it is better to put the UV unit upstream of the filters so the water flow through the UV can be easily adjusted. The constriction of the UV unit may not allow enough water to pass through it in a gravity-fed setup.

Getting the proper size UV and using the proper flow through it is essential to controling green water. The size of the unit is usually based on the total wattage of the bulb(s). Most manufacturers will also rate the unit by flow rate. Some brands work better than others at the recommended flow rate and the density of the green water (how green is it?) will influence the results. To understand how to purchase the correct size unit and get the optimum flow through it, you need to understand a little bit about how a UV unit works.

Most units have a cylindrical housing, about 3 to 6 inches in diameter, with an inlet port on one end and an outlet port on the other end. Inside that cylindrical housing sits a glass tube or glass sleeve with seals to keep water from entering the sleeve. Inside the glass sleeve sits the UV lamp. The UV lamp is long and skinny – a little over one-half inch in diameter. Lamps with higher wattage are usually longer, so the glass sleeve and the cylindrical housing are also longer. Water entering the cylindrical housing at one end has to pass by the UV lamp encased in the glass sleeve as it makes it way to the outlet at the other end. Some units use fancy baffels to make sure there is a uniform flow through the unit.

Some UV manufacturers use cagey math to rate their units. If the manufacturer says a unit is rated for a certain flow, check what the UV dosage is at that flow. Many will base recommended flow on a dose of 15 microwatt-seconds per square centimeter because that dose will kill many bacteria. However, it usually takes at least 22,000 microwatt-seconds per square centimeter to kill algae. Some say that the dose for algae should be 40,000 mw-s/cm2 to accommodate less-than-clear water and the degradation of the UV bulb over time.

That 22,000 microwatt-seconds per square centimeter dosage means 22 watts (= 22,000 microwatts) of UV radiation bombards a square centimeter for one second. You could also use 11 watts of radiation bombarding a square centimeter for 2 seconds and get the same dosage. Therefore, if you slow down the flow, you increase the UV dosage. You can also increase the dosage by increasing the wattage of the bulb. Since single cell algae reproduce fairly rapidly, you should subject the entire pond volume to the UV dosage every few hours.

The unit of measure is square centimeters (a surface area) not cubic centimeters (a volume). The effective depth of penetration of the UV rays depends on the amount of turbidity (such as algae and other solids) which will absorb the radiation. Dissolved iron and magnesium also seem to absorb a lot of UV radiation, as do many other compounds which contribute to general hardness. This effective depth is usually a couple of centimeters—less than one inch. Recommended flow rates from the manufacturer usually assume the water is very clear and low in dissolved solids.

The dosage needed to kill big stuff like parasites is about 10 times higher than what is needed to kill algae. This assumes the parasite happens have a free-living form which would pass through the UV unit. Most parasites are either on the pond bottom or on the fish.

If you know the wattage and length of the bulb, the diameter of the glass sleeve the bulb sits in, and the flow rate of the water going past the bulb, you can calculate the dosage. This gets a little cumbersome though. An easier approach is to take the manufacturer’s recommended flow rate and correct it, if needed, for killing algae instead of bacteria. If the manufacturer says the unit is rated for 20 gallons per minute, then look for information on the UV dosage that will be delivered at that 20 gpm flow rate. If a dosage is not given by the manufacturer, then they are probably using a dosage of 15,000 mw-s/cm2. If you want a dosage which is twice as high, say 30,000 mw-s/cm2, then the flow you use through the unit should be half as much, only 10 gallons per minute.

The common bulb sizes are 8, 15, 25 and 40 watts. For larger units, they generally just add more 40-watt bulbs. You want to know the bulb wattage, not the wattage used by the unit. Just like a normal florescent light, he bulbs need a ballast to operate. The ballast draws more electricity than the UV bulb consumes. A UV unit with a 40 watt bulb may use 55 watts of electricity. The extra electricity escapes as heat from the ballast.

The UV output of a bulb degrades over time. They usually recommend that you replace the bulb after 8 to 12 months of continuous use. If the unit is designed for a dosage higher than the minimum dosage required to kill algae (>22,000 mw-s/cm2), then the unit will put out an adequate amount of radiation to kill algae over a longer period of time. Many people have a UV unit, but do not use it all the time – only when there is a green water problem. When using a UV unit in this way, it is important to keep track of how long the bulb is turned on so you will know when it is time to change the bulb. Some high-end units have an hour meter that records how long the bulb has been in use.

You cannot tell if a bulb is exhausted by looking at it. In fact, you should NEVER look at a UV bulb when it is turned on - not even for a second. The UV radiation will burn your eyes. You do not even know it has happened until your eyes start to burn several hours later. It will hurt and could cause serious damage.

The glass sleeve is also an important consideration in UV unit construction. There are some inexpensive UV units which do not have a quartz sleeve between the bulb and the water. A problem with these is that the water in contact with the bulb keeps the bulb cool which reduces it’s UV output. The quality of the glass sleeve is important. A good UV unit will have a sleeve of quartz glass since quarts absorbs less UV radiation than silica glass. I am told that there are various grades of quartz glass with the better grades absorbing less UV than the poorer grades. Quartz glass is not cheap.

Finally, cleaning the quartz sleeve is important. When the surface of the sleeve in contact with the water becomes dirty, it absorbs some of the UV radiation. Some UV units used in industrial aquaculture have a little gizmo which slides back and forth to clean the surface of the sleeve without having to dismantle the unit. For most units used in koi ponds, the sleeve has to be removed from the unit to clean it. This is not only time consuming, it can be the root of other problems such as wearing out the O-ring seal and causing the unit to leak. More often than not, when bulbs and sleeves are accidentally broken it happens when the unit is being taken apart for cleaning.

Those bulbs can be a real nuisance to maintain. The end caps fall off, the connections become corroded, the little wire dangling precariously alongside the bulb breaks, tap the bulb once on a hard object and the glass breaks. Yet, to get the maximum effect, you have to handle and remove the bulb and sleeve for cleaning on a regular basis.

One more little consideration. When calculating the flow needed to get the dosage you require, you have to be careful not to slow down the flow too much. If you use only half of the rated flow, its probably OK. However, if you slow down the flow to 10% of the rated flow, then you need to make sure the water is not being heated up too much by the prolonged contact. If the water gets too hot, the plastic will warp and the thing will start to leak at the seals.


As noted above, some people really like green water. Anyone raising koi or goldfish fry, or managing a mud pond for production purposes tries to promote green water. Ponds are fertilized with manure or agricultural chemicals to promote green water until such time as the feed input is sufficient to maintain the algae.

Green water is the base of a food chain which feeds small water fleas and other zooplankton. Koi and goldfish fry feed on the zooplankton and larger fish feed on the worms, insects and other forms which are nourished by the green water and zooplankton. In addition, goldfish and koi seem to be able to consume the green water directly, despite the small size of the algae cells. It is thought that algae cells are ingested as they reach the end of their short life span, sink to the bottom, and develop into clumps of cells held together by bacteria slime. The protein content of algae is actually quite high and they contain many types of pigments which intensify the color of fish.

Spirulina, a type of single cell algae, is often added to koi and goldfish feeds as a source of high quality nutrition and as a color enhancer. Much of this Spirulina is grown, processed, and packaged on aquaculture farms adjacent to the Kona airport on the Big Island.

A recent trend in Singapore, where there are a lot of serious goldfish hobbyists, is to cultivate green water in the home goldfish aquarium. These people are using 20 to 100 gallon aquaria with no filtration or gravel substrate – just a bare tank with an air stone or two. Fairly intense artificial lighting is used overhead. Initially, the aquarium water may be inoculated with green water if it does not develop on its own. When the goldfish are fed, they produce carbon dioxide and ammonia as waste products. These waste products are just what the single cell algae needs to multiply and increase their numbers. The algae take up carbon dioxide and ammonia and release oxygen through photosynthesis. Weekly, 50 to 70% of the aquarium water is discarded and replaced with new water. This keeps the algae growing rapidly and maintains water quality.

Compared to a clear-water aquarium with biological filter, the benefits of this green water approach to goldfish keeping are said to include the following:

  • The water quality is improved.

  • The goldfish appetite and growth is improved.

  • The goldfish health is improved.

  • There is little or no incidence of swim bladder abnormalities.

  • The goldfish color is improved.

  • The development of head growth on ranchu, lionhead and oranda is dramatically improved.

  • There is no nitrate build-up.

  • Goldfish stress is reduced due to the lower visibility.

  • It is a free source of nutritious, color-enhancing food.

  • The color of the water is an accurate indicator of fish disease. If the water turns from green to brown, it means there is some disease agent at work. (Editor’s note: This phenomena is poorly understood and one has to question whether the cause and effect are reversed.)

  • There are disadvantages and potential problems associated with the green water approach:

  • During the day when the algae are removing carbon dioxide, the pH increases. Carbon dioxide is a weak acid. At night, when carbon dioxide levels rise again, the pH falls back to baseline levels. Most people feel that these gradual pH fluctuations are not stressful and some feel they may even make the goldfish stronger.

  • During the day, algae produce more oxygen than it consumes. But during the night or whenever the lights are off, oxygen consumption by algae continues so air stones are needed to maintain oxygen levels at night.

  • There are reports of goldfish getting gas bubble disease (gas embolisms forming in the fins and gills) due to supersaturated oxygen. An air stone should drive off excess oxygen and prevent this problem.

  • You can’t see your goldfish clearly in green water.

  • Another developing trend in Singapore is keeping two goldfish tanks. Half or more of the goldfish are kept in a green water tank, tub, or pond – usually outdoors or on a lanai where there is natural sunlight. The rest of the goldfish are kept indoors in a clear-water display aquarium with biological filtration. Every few weeks or months, the fish in the clear water are moved to the green water and fish from the green water are moved to the clear water display tank. This technique maintains goldfish growth, health and color while allowing you to enjoy the display tank.

    Sound familiar? For decades, serious koi keepers in Japan have been renting pond space during the summer for some of their fish. While in the green water mud pond, the koi grows rapidly and it’s health and color are restored. KHV is changing these traditions, but there is no doubt that koi benefit from green water as much as goldfish do. Will having a clear water koi display pond and a green water koi conditioning pond at home be the next trend?