Pond Ultraviolet Lights

Author: Pets Parade Team

Why do I need an Ultraviolet light on my pond?
Ultraviolet lights, or Ultraviolet clarifiers as they are more correctly called are a valuable tool to maintain clear water in goldfish or Koi ponds. Most garden ponds will at some time suffer with green water algae this is not usually harmful to fish but looks unsightly and obviously makes it difficult to see the fish. Although it is possible to obtain clear water by achieving a natural balance in the pond, this usually takes a long time and is much easier to achieve in a large pond, particularly a pool that is well planted. For most small to medium ponds it is difficult to maintain stable water conditions and keep the pond water clear of algae with out the use of chemicals or an Ultraviolet light. Koi ponds provide a slightly different challenge from Gold fish ponds although they are often a large size with stable water conditions they seldom have many plants. Koi ponds also tend to have large fish and a heavy stocking density, this leads to a large amount of waste being produced which acts as a fertiliser for green water algae (and blanket weed).


How do Pond Ultra violet lights keep pond water clear?
The ultraviolet light unit consists of the electrical control gear and a central housing that contains a quartz glass sleeve with the Ultraviolet tube inside it. Water passes through the housing and around the glass sleeve where it is exposed to the Ultra violet light. The Effect of the Ultraviolet light is to damage the cell walls of the algae and cause it to clump together. The algae then pass directly into the filter system or back into the pond where it then passes to the filter as water is pumped around the pond.

The effectiveness of the Ultraviolet light at clearing the pond water is dependent on several factors: the strength of the Ultraviolet tube itself, the closeness of the Ultraviolet tube to the water (this depends on the diameter of the housing, how "dirty" the water is and the speed at which the water is pumped through the unit.

Factors that affect Ultraviolet lights
The strength of the ultra violet tube is important in killing algae and obviously the larger the pond the stronger the tube will need to be in order to be effective. Pond Ultraviolet lights are typically available with tubes that vary from 4W up to 55w. Bigger units are available for large Koi ponds but these consist of multiple tubes usually two or more 30w or 55w UV tubes connected with a plastic manifold.
When selecting an ultraviolet light for your pond it is best to use the manufactures recommendation as a starting guideline but to apply a healthy dose of scepticism to the claimed figures.
The recommended pond size given by the manufacturer is the absolute maximum that can be kept clear of algae in ideal conditions, i.e. very little direct sunlight and a low stocking density of fish. In most situations to get a realistic figure for the volume that will be effectively treated it is best to subtract around 20% from the maximum quoted pond size. For example a pond Ultraviolet light that is recommended for up to a thousand gallons will probably only be suitable for eight hundred gallons in most real world situations.

Ultraviolet light waves rapidly lose strength as they pass through water. All things being equal the smaller the diameter of the tube containing the pond water as it passes through the Ultraviolet unit the more effectively the algae will be killed. There are many variations on the actual design of the ultraviolet units available for ornamental ponds. Some manufacturers claim their design produces turbulence in the water flow that better exposes the algae to the full strength of the Ultraviolet rays. The variation in design explains why different manufactures claim their ultraviolet can be used on a bigger pond even though it use the same wattage bulb as one for a smaller pond from a different company.

If the water passing through the Ultraviolet light is "dirty" I.e. with solid particles in it, this will reduce its effectiveness because some of the algae will be shielded from the Ultraviolet by the particles. For this reason Ultraviolet lights are much more effective in keeping a pond clear if used in conjunction with a filter box. The filter box will remove solid particles as well as carrying out biological filtration of the pond by breaking down the fish waste.

The new fish keeper may wrongly believe that the purpose of his filter is to clarify the water. However the biological filtration in a pond is solely for the production of nitrifying bacteria to break down harmful fish waste. Water clarity is usually influenced by the final product of a biological filter which is nitrate which acts as an excellent fertiliser. Together with any phosphates or other imbalances, nitrate will encourage the growth of single celled algae causing green water

It is important to remember that pond Ultra violet lights will only kill off algae that is passing through them and therefore will have no effect on any blanket weed that is anchored to the side of the pond. Ultraviolet lights should be turned off for the duration of the treatment if you need to add any medication to your pond, as they will reduce the effectiveness of the treatment as it passes through the U.V.

Causes of green water in the pond
Green algae need a combination of sunlight and nutrients in order to grow. Garden ponds usually satisfy these requirements extremely well. Waste from the fish and from any uneaten food produces a regular supply of Nitrates and Phosphates this is utilised by the algae to produce rapid growth and unsightly green water.

The use of the correctly sized Ultra violet light will keep the water clear, but it is important to regularly test your pond water for Ammonia, nitrites and nitrates. All these substances are harmful to pond fish to varying degrees. All dissolve in the water and so cannot be seen with a good ultraviolet clarifier it is perfectly possible to have crystal clear water that is still toxic to your fish.
Remember how clear the water is may be important to you, but your fish are affected more by the amount of dissolved waste products in the water. A good quality test kit is the only way to be sure your Goldfish or Koi are living in healthy water conditions.