How fast can a UV clear up a green pond? Back to top

Typically, if the UV is sized correctly and the flow< rate matches the manufacturer's suggested rate , most green water ponds can be cleared in three to five days. If it takes longer than three to five days, the flow rate may be incorrect or your UV is too small for the pond. If you're certain that you have the correct UV size for your pond, consider slowing down the flow rate about 20% and wait a few days to see if the water clears. Repeat if necessary. If you install a UV that is larger than what is required for your pond, it may take less than three days to clear up a green pond, and can sometimes take as little as one to two days.

How do I figure out which UV I need? Back to top

First review the UV manufacturers recommended pond size. Each manufacturer recommends different UVs for different size ponds. Then you'll need to consider the flow rate. Larger UVs can handle faster flow rates, but they cost more. By buying a UV that can handle your total systems flow rate, you may end up treating the entire pond more than once, every three to four hours. This also makes plumbing easier because you won't need to install flow rate valves, therefore taking up less room when installing your pond equipment.


You're building a 4,000 gallon pond and need to figure out which UV will provide the best results for your pond and ensure that your water remains crystal clear. Since your total pond volume needs to be circulated through yourUV at least once every 4 hours, the following equation will provide you with the correct Flow Rate (FR) for your pond. You can than compare your Flow Rate with the UV manufacturer to see which UV will provide the best results for your pond.

Gallons of water in your pond / Time for the pond volume to be circulated = Flow Rate

4,000 gl / 4 hrs = 1000 gallons per hour

The 1000 gallons per hour is your flow rate. You than compare this number to a UV Manufacturers data sheet. For example, Emperor Aquatic offers a 25 watt UV with a max flow rate of 788 gph and 40 watt UV with a max flow rate of 1,574 gph. Since you need to ensure that your pond is circulated enough times you would go with the 40 Watt UV. Using the above equation will help to ensure that you choose the correct UV for your pond, providing the best results and crystal clear water.

However, if your flow rate is faster than 1,574 gph and you choose to not configure your pond with flow rate valves, then you'll need to find a larger UV with more watts and a greater flow rate.

Example; Your 4000 gallon pond is setup with a pump and filtering system that flows around 4000 gallons each hour, you review the manufacturers specifications and find that the Emperor Aquatics 120 watt UV works well because it is rated for a max flow of 4,600. The advantage is that it's easier to install but the unit and the lamps costs more. The larger UV also takes up more room.

Climate can also play a factor when deciding which UV to purchase for your pond. With the sun beating down on your pond it increases the algae in a pond and helps it flourish. If your pond is exposed to a lot of sun or if you have a shallow pond it might be wise to consider the next size up. By purchasing a UV with a higher flow rate it will help to ensure that your pond will stay crystal clear, even if your pond is in the hot sun all year round.

Which way should the water enter and exit the UV? Back to top

The end of the UV that has a power cord is usually considered the top or the end in which the water exits. A good tip to remember is that if you install the UV horizontal, then be sure to point the ports up. This increases the ability for the air to get out when the water is flowing through.

Is Steel better then plastic? Back to top

While steel is stronger, it normally offers very little benefit. Once a UV is installed, its typically untouched until it comes time to replace the lamp. So strength has very little value, even in freezing climates. Most steel UVs also have plastic connections so when the UV freezes, the weak link may still require getting a new UV.

Do Stainless steel chambers offer reflection benefits? Back to top>

Does myth help? How about marketing spin? Its sounds nice and yes reflection of light is possible, but as UV travels, it continues to get weaker and weakerÖ If it did reflect back, itís going to be much weaker then it was prior to reaching the reflective surface. UV labs discount the benefit of reflection.

How does a UV clear the pond water up? Back to top

The specific wavelength used in UV lights is selected for the sole purpose of being the correct one to kill the reproductive cell or DNA of the algae cell. Once killed or even crippled, it is doomed to death. This makes having the correct flow rate all the more important. If the water is passing too fast, then the light canít kill the cell. It requires a little suspension time. How do I know if I need to replace the UV lamp? First check to see if your UV has a glow. Itís easier to see at night. During the first year of usage 24/7, the lamp will continue to get weaker. At around 13 months of usage (Or 9000 hours), the lamp is considered to be at the end of life. While it may still glow, it is too weak for performing the job required. Replace the lamp once a year to keep the UV performance peak. How easy is it to replace the UV lamp?

Most UVs require accessing the power cord end that plugs into the lamp. Once accessed, unplug the old lamp, plug in the new lamp, and reassemble. Some UVs are easier, and can easily be replaced by simply unthreading a large plastic nut that the power cord passes through. While other designs may require removing a cover or a lid that keeps the power supply parts waterproof. This then exposes the power cord end that the UV lamp is plugged into. Care should be given to not touch the glass of the new lamp. The oils on your hand may shorten the life of the lamp. Are UVs water proof?

The housing is usually waterproof and can typically be exposed to weather. Power supplies mounted to the cord may have a pinhole to permit the heat to escape as needed. Verify that the UV doesn't have a hole before considering any condition that may short out the unit from water. The UVs that have access covers to the power supplies are only waterproof from normal weather and should not be submerged.

Where do I install my UV? Back to top

You should install your UV after the filter. There are a two big reasons for this. First and foremost your UV will work faster if you put it after the filter. Secondly, if you put the UV before the filter you risk the chance of small rocks and other debris breaking the quart sleeve.

Do I have to turn my UV off when I clean my filter? Back to top

Only if your UV chamber is drained of water or if the water will be still for more than 30 min. If you leave the UV running for more than 30 mins in still water it can cause extreme damage to your UV.

What should I expect after my UV is installed? Back to top

UVs on average take 3-5 days assuming that you install the UV after the filter. If you do choose to install the UV before the filter it can take between 7-10 days. However, UVs do not destroy the green algae on the sides or the bottom of the pond. This algae is common on ponds and completely normal.

Whatís the difference between a sterilizer and a clarifier? Back to top

UVs are considered a sterilizer because they keep your pond water crystal clear. In addition to clear water it will help control the bacteria in your pond, keeping your fish healthier and decreasing their chances of disease. Clarifiers, which are often in powder form, don't completely clear up your water and leave a green tint to it.

Do I install my UV vertically or horizontally? Back to top

You can install UV in either direction. But if you install your UV horizontally be sure to keep the nozzle facing upward.

What should my flow rate be on my pond? Back to top

To ensure the highest quality water your flow rate should turn over the volume of water once every hour or hour and a half.

Should I bypass part of my water through the UV and the rest back to the water supply? Back to top

No, because UVs can handle high flow rates and by sending all of your pond water through the UV you help to ensure the highest water quality of your pond.

Does the UV light have any residual effect to the fish? Back to top

UVs don't have any residual effect because it only affects what passes through the chambers of the UV as it passes through.

Does the UV kill good bacteria? Back to top

UVs kill any and all bacteria that passes by the light, including good bacteria. But most beneficial bacteria tend to reside in your filter as well as on the sides or bottom of your pond, where they remain out of harm from the UV.

How often should I change my lamp? Back to top

Each manufacturer recommends different lengths of time depending on what they feel is best for their particular UV