Should the UV lamp be turned off during a GH or KH treatment of the pond?
At the Nodebais counter, the question comes up every spring, often asked by someone carrying a bucket of mineral salts under their arm: "I need to raise my KH, should I turn off the UV?" The answer sometimes comes as a surprise. For a simple addition of GH or KH salts, the UV-C can generally continue running: these minerals are not destroyed by the radiation. It is other products, often added during the same week, that require it to be turned off. The key is knowing which ones.
— The short answer
Products that raise hardness (GH) or buffering capacity (KH) are dissolved mineral salts, including calcium, magnesium, and carbonates. UV-C does not affect them: there is no need to turn off the lamp. However, turn off the UV when adding live bacteria, a liquid algaecide, or medication, and always follow the duration stated in the product instructions. If your "GH/KH" product also contains bacteria or organic trace elements, read the label: that is what determines the answer.
What a UV-C actually does in the circuit
A UV-C clarifier is a tube through which water flows around a lamp emitting at around 254 nanometers. This radiation damages the genetic material of microscopic organisms passing through the device: single-celled algae, free-floating bacteria, and certain parasites in their swimming stage. Suspended algae then clump together and are trapped by the filter. UV therefore does not "filter" anything and does not alter the water's mineral chemistry.
Calcium, magnesium, and bicarbonates, which make up hardness and buffering capacity, are dissolved ions. They are not living, have no DNA to damage, and the UV doses from a pond device do not transform them. For this reason, GH or KH salts can be added with the lamp on. Water quality manuals in aquaculture, such as Boyd and Tucker's, also treat alkalinity and hardness as purely chemical parameters, adjusted through mineral additions.
The breakdown, product by product
| Product type | Turn off the UV? | Why |
|---|---|---|
| Pure GH or KH salts | No, generally | Dissolved minerals, insensitive to radiation |
| Start-up or maintenance bacteria | Yes | Free-floating bacteria passing through the UV are destroyed before colonizing the filter |
| Liquid algaecides | According to the instructions, often yes | Some formulas are degraded by UV or work better without it |
| Fish medications | Yes, unless otherwise stated | Several molecules are sensitive to UV light |
| "Combined" products (minerals + bacteria) | Yes | The living component dictates the rule |
Noga, in his reference work on fish diseases, points out that a treatment's effectiveness depends on its actual concentration in the water throughout the planned treatment period. A UV lamp or activated carbon running at the same time can reduce this concentration without anything being visible. Hence the almost systematic instruction from medication manufacturers: turn off the UV and remove the carbon.
IchiPond KH+ 1 kg – Aquatic Science
To gradually raise the buffering capacity of rainwater or very soft water, without affecting the UV. Live pricing and add to cart:
The spring treatment trap
The classic scenario looks like this. In mid-April, the water warms up, the filter is cleaned, the UV is switched back on, bacteria are added to restart the biological process and, at the same time, KH salt is added because the test showed that the buffering capacity was too low. Everything is added on the same Saturday. Result: the bacteria repeatedly pass through the lamp, the filter starts poorly, and the owner concludes that “KH was useless” or that “the bacteria do not work.”
It is better to separate the steps. First correct KH, with the lamp on, in several small doses spaced a few days apart. Once KH has stabilized, turn off the UV, add the bacteria, and leave it off for the period indicated by the manufacturer, often around one to two days. Well-buffered water is also beneficial to nitrifying bacteria, which consume alkalinity as they convert ammonia.
Why KH deserves our attention
In a pond filled with rainwater, or in a soft-water region, KH can fall very low. Yet it is what buffers pH fluctuations between day, when plants and algae consume CO2, and at night. Wetzel describes this carbonate system well: the fewer bicarbonates there are, the more the pH fluctuates. For koi, these fluctuations are a source of stress. A KH maintained within a moderate range, around a few German degrees, is generally enough to stabilize things.
GH mainly concerns calcium and magnesium. It is rarely too low in Belgian tap water, but it can be low in a pond filled only with rainwater. Measure before adding anything: correcting “blindly” is the main cause of excessive dosing we see in the shop.
AquaActiv Quicksticks 6 in 1 – Oase
Test strips for monitoring GH, KH, and pH before and after each adjustment. Live pricing and add to cart:
The right procedure
- Measure GH, KH, and pH in the morning, before making any adjustments.
- Read the product label: minerals only, or a blend with bacteria and additives?
- Minerals only: leave the UV on, dose in small increments, and measure again after a few days.
- Bacteria, algaecides or medication: turn off the UV before dosing, follow the specified shutoff period, then turn it back on.
- Note dates and doses: after one season, you will know how your pond behaves.
One practical detail: turning off the UV does not mean turning off the pump. Circulation and oxygenation must continue, especially when you add bacteria that consume oxygen. In many installations, the UV is connected to its own outlet; if this is not the case for you, an outdoor power strip with separate switches makes life easier.
AquaActiv BioKick 200 ml – Oase
Starter bacteria to add with the UV off, once the buffering capacity has been corrected. Live price and add to cart:
Also read
The case of bacteria in detail: Should you turn off the UV when adding bacteria to a pond?.
READ THE ARTICLEUnsure about a specific product? Send us a photo of the label and your latest GH, KH and pH measurements: at Aquiflor in Nodebais and on Foudebassin, we will tell you whether to turn off the UV and for how long.
Frequently asked questions
— Can UV radiation lower the KH?
No, not directly. The KH mainly decreases due to nitrification, rainwater and biological activity. The UV has nothing to do with it.
— How long should I turn off the UV after adding bacteria?
Follow the manufacturer's instructions. It is often one to two days, allowing time for the bacteria to settle in the filter.
— Can I add GH and KH on the same day?
Yes, if the products are compatible, but increase the dose gradually and measure again after a few days rather than correcting everything at once.
— Should the pump also be turned off during treatment?
No. The pump ensures circulation and oxygenation. Only the UV, and possibly the activated carbon, should be taken out of the circuit depending on the product used.
In summary
- GH and KH salts are minerals: the UV-C can remain on.
- Bacteria, algaecides and medications: turn off the UV, for the duration specified in the instructions.
- Correct the KH first, then add the bacteria in two stages.
- Measure before and after: do not make blind corrections.
Sources and references
- Wetzel R. G., Limnology: Lake and River Ecosystems, 3rd edition, Academic Press, 2001
- Boyd C. E. et Tucker C. S., Pond Aquaculture Water Quality Management, Kluwer, 1998
- Noga C. J., Fish Disease: Diagnosis and Treatment, 2nd edition, Wiley-Blackwell, 2010
- Manufacturer technical datasheets: Oase (AquaActiv, Vitronic, Bitron), Aquatic Science (IchiPond)
- Aquiflor / Foudebassin consulting experience, Nodebais (Belgium)
Last updated: September 29, 2026.
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