Information on ozone (ponds and aquaculture)

An introduction to ozone

Ozone sounds familiar to most people, whether it’s because of the ozone layer or the smell of some printers. However, a lot of people don’t know that ozone can be used to clean water as well. Ozone has been used for years in the production of drinking water as an alternative to chlorine. It’s use has also increased in pools because chlorine can cause irritations to lungs and eyes. When ozone is correctly used, it can solve those problems and still keep the water clean. Last couple of years, ozone has also made an advance in fish farms, zoos and (koi)ponds. The use of ozone is an excellent addition to your filter, but not a replacement for a filter! A lot of information on ozone can be found on the internet. Unfortunately, there’s also a lot of misinformation. In the text below, we’ve tried to explain the basics of ozone the best we can.

What is ozone?

Ozone consists of three oxygen atoms (O3) and is produced out of oxygen. Oxygen consists of two oxygen atoms (O2). Converting oxygen into ozone can be done in two ways: with UV light or using high voltage. The latter is used most often because the maximum ozone concentration is much higher than when UV light is used. Plus, the amount of electricity used per gram of ozone is much lower. Ozone is highly instable and will want to lose one oxygen atom as quickly as possible to fall back into oxygen.

What’s ozone used for?

Ozone is used to remove chemicals from water that are normally very hard to get rid of. For example: dyes, nitrite, medicine residue and pheromones. It also kills bacteria, parasites and viruses way more effcective than a UV light ever could. However, it only reacts with the chemicals, parasites, bacteria, etc. that pass through the reaction vessel. This way, the bacteria in your filter you want to keep won’t be affected. Pure oxygen is the only thing that remains of the ozone after it’s done it’s job!

What does an ozone installation consist of?

A good ozone installation consists of a couple elements with their own specific function.

Ozone generator:

The ozone generator converts air (this contains oxygen) into ozone. The capacity of an ozone generator is measured in grams of ozone per hour. This often varies between 500 mg and a couple grams in ponds.

Air dryer / oxygen generator:

Using an aur dryer or an oxygen generator is highly recommended to dry the ingoing air of an ozone generator. Moist air can cause the ozone yield to be reduced by more than 50% and causes the formation of nitric acid in the ozone cell, which lowers the lifespan of the ozone cell. An oxygen generator also makes sure a lot more ozone is produced.

Ozone reactor:

This is actually a collective name for all the products used to make ozone react with water. A good ozone reactor mixes small ozone bubbles (2 to 5mm) with water because this is the most efficient way. The industry often uses a static mixer or a venturi to mix ozone with water, these achieve the highest return. The downside of this method is the high energy consumption because very powerful pumps are needed to make them work. Vessel-shaped ozone reactors with air stones to mix the bubbles in are the go-to method in zoos and the aquaculture. This is sometimes combined with a skimmer. This has one big advantage: the skimmer removes part of the chemicals that were oxidised by the ozone.

Surplus ozone destroyer:

Most ozone reactors have a ‘surplus air outlet’, this doesn’t only let air out, but ozone too. This gas stream shouldn’t just be released in your filter area because ozone is harmful to both humans and animals. An ozone destroyer is used to neutralise the ozone that’s present in the outgoing air. The destroyer converts the ozone into oxygen.

Redox controller:

The purity of your water can be expressed in the redox potential (mV). Ozone makes this potential go up. Too high a redox potential is harmful to your fish, which is why a redox controller is used. It works just like a thermostat on a central heating boiler. The user sets the desired potential and the redox controller makes sure the ozone generator shuts off when the potential gets too high. It also turns it back on when the redox potential drops below the desired value.

The proper use of ozone.

Proper use of ozone starts with an appropriate ozone installation and a couple guidelines.

Redox value:

As we’ve mentioned before, the redox value is an indication of the purity of the water. A redox value of 275mV is the recommended maximum for a pond.

The arrangement of the reactor:

It’s very important to not let the outlet of your ozone reactor end in your basin/pond. Some ozone might still be present and this can harm your animals. It’s best to choose the pre-filter, last filter chamber or the plant filter as a place for the outlet. It’s sometimes recommended to place a UV light behind the outlet but this is highly inadvisable. UV light converts ozone into hydrogen peroxide and ozone and hydrogen peroxide will form OH radicals. Both chemicals are very harmful to your fish.

A couple persistent misunderstandings about ozone:


Some websites claim ozone breaks down ammonia, which isn’t completely true. Under certain conditions (pH), ozone will break down a tiny bit of ozone. Such a small amount that it’s not even worth mentioning. Ozone does convert nitrite to nitrate!


We often receive questions about removing algae with ozone. Killing and removing algae will barely happen with the ozone installations usually found on ponds. It is possible but about 10 times more ozone is needed than normally used on ponds. Besides that, the contact time is often too short. A contact time of 3 minutes or more is needed to kill algae.


Some companies claim ozone removes phosphate, unfortunately it doesn’t.

Small ozone bubbles are the most important factor to properly mix ozone.

This statement assumes the contact area to exchange ozone is bigger when the ozone bubbles are smaller. This is true, but a couple other factors should be considered. It comes down to the fact that bubbles between 2 and 5 millimetres are most beneficial for good exchange. When they’re smaller than 2,5 millimetres, the movement of ozone molecules within the bubble will start to matter, but this is too complicated to explain here. However, the statement that smaller bubbles are always better definitely isn’t true!

Removing surplus ozone with UV.

This is highly inadvisable. UV light converts ozone into hydrogen peroxide, ozone and hydrogen peroxide form OH radicals. This method is used in the industry, not to remove ozone but because OH radicals are better at removing some chemicals than ozone. This does indicate how seriously OH radicals should be treated.

Ozone decays within a couple minutes.

Ozone decays relatively fast, but how long it takes depends on the circumstances and the amount of chemicals that are to be oxidated. For example, in pure water with a temperature of 20 degrees Celsius and a pH of 7, most of the ozone will have decayed in 30 minutes. This will happen faster in a pond because of the pollution. However, when you mix too much ozone into the water, too little chemicals will remain to be oxidised. In this case the surplus ozone will react with the next chemicals that can be oxidised, which will probably be in your pond or filter. Unfortunately, ozone sees no difference between pollution, bacteria or fish and it will just react with the first thing it comes into contact with.

An ozone generator can’t be too big.

We come across this statement quite often, people often assume the redox controller makes sure the redox value doesn’t get too high, this is almost always true but it is still wise to pick an ozone generator that isn’t too big.

Ozone increases the amount of oxygen.

People say an ozone installation increases the amount of ozone, which is partly true. Air goes through the generator to make ozone. This air is partly converted into ozone, which will react in the reactor or mixer with the chemicals in the water. This does increase the oxygen levels because you aerate the water at the same time. But directly mixing the air into the water would do a much better job at adding oxygen than an ozone generator would. Another thing worth mentioning is that ozone will react with the chemicals in the water. A couple of these chemicals would otherwise be oxidised by oxygen, such as the ones part of the rotting process. Because ozone now reacts with those chemicals, the oxygen consumption of the pond will decrease. From that angle, ozone is oxygen increasing.

Disease pressure.

Ozone will react with the bacteria and parasites in the water. In normal use, this should decrease the disease pressure in your pond, but not to a harmful extent. When the redox value is too high, the disease pressure could get low enough for your fish to loose resistance against diseases. In a good ozone installation, the ozone will only react with the chemicals that pass through the reactor or mixer. When properly used, the filter bacteria won’t be damaged.

Required amount of ozone.

The amount of ozone that’s needed in a pond depends on many factors like the filter, the type of ozone reactor, fish population, etc. In general, 1 gram per 20m3 is enough, provided that a good reactor or mixer is used. Higher values are sometimes advised, up to 3 grams per 20m3! This isn’t just unnecessary but also dangerous because of the surplus ozone that can be formed.

Specified yield of an ozone generator.

The ozone yield of ozone generators is given in grams of ozone per hour. Most manufacturers only provide that piece of information, without specifying the measuring conditions. This is why an ozone generator can be sold as one that produces three grams per hour, when it actually produces one. How can this happen?
In this case, the three grams that are specified are measured under optimal conditions, a couple options:
The ozone yield was measured with oxygen as a supply gas. On normal air and under optimal conditions the generator will be able to produce about 65% of the specified amount, which comes down to roughly 2 grams.

Another possibility is that the ozone yield was measured with air as supply gas but it was thoroughly dried and cooled. The dew point (dryness) of the air is of great importance to the ozone yield. Ozone yields are sometimes measured at a dew point of -60 degrees Celsius, which can only be reached with highly advanced air dryers. In this case the ozone generator definitely won’t generate those 3 grams.
So please, when you’re looking to buy an ozone generator, pay attention to the measuring conditions and not just the ozone yield!

Lastly, a warning:

Ozone is a dangerous substance that should be handled with care, it’s also harmful to humans. Make sure that you never smell ozone, something’s wrong with your installation if you do. A ‘trick’ people sometimes use is smelling the outlet of the ozone generator to check whether it’s still working. PLEASE DON’T DO THIS!
As you see, there are quite a couple misunderstandings about ozone. Should you have any questions about the uses and applications of ozone, please don’t hesitate to contact us Tol Watertechniek or one of our dealers.

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