Carbon dioxide pond problems can be controlled by waterfalls, aeration and uv lights

Carbon dioxide in pond water results from a number of sources including:

1. waste products decaying at the bottom of the pond

2. respiration by pond inhabitants .... fish, insects, plants, algae

3. growing pond plants help the oxygen balance in ponds to the benefit of its inhabitants and the bio filter

Of course all of us think first and foremost of oxygen in pond water as being essential and of course this is very true. Carbon dioxide is also of critical importance. Oxygen and carbon dioxide also work in concert with each other.

In simple terms as carbon dioxide levels increase in a pond then oxygen levels tend to decrease. This follows a natural pattern as can be seen in the sketch. Here you can see that from dawn to dusk (daytime) oxygen levels in pond increase and decrease again from dusk to dawn (night). Carbon dioxide concentrations in water act in reverse - falling during daylight hours and rising during the night. Oxygen levels are at their highest at dusk and carbon dioxide levels are highest at dawn.

Beware algae blooms in ponds

Let's equate this to what living organisms do in the pond. Think of the flora and fauna as exhaling carbon dioxide while inhaling oxygen during the night.
From this you will agree that dawn is the critical time in a pond. Often people wake up to find dead fish and wondered what happened. Such deaths could be associated with very low oxygen levels coinciding with high carbon dioxide levels. When ponds are full of suspended algae such problems can arise.

If there is a very small amount of algae bloom in the pond then you will find oxygen and carbon dioxide levels will not change significantly between early morning and late afternoon. On the other hand dense pea soup type water will show very significant variations.

Seasonal Impacts upon carbon dioxide problems

The first thing to realise is that oxygen concentrations are highest in winter because water is cooler. This means the concentration of oxygen can be higher. Because oxygen concentrations are high the oxygen reserve is not depleted so quickly during the night. Plant and animal life has also slowed down significantly.

On occasions fish in ponds with no fountains or waterfalls may look listless in winter due to carbon dioxide levels being excessive but this is normally associated with a long run of calm dull days - in these circumstances there is no natural wave action to allow oxygen to be transferred to the water. The problem quickly sorts itself out when windy and bright weather returns.

In summer water can hold much less oxygen and the animal and plant life (algae) is also thriving due to higher temperatures along with more nutrients in the water associated with feeding fish. The living organisms are therefore emitting more carbon dioxide in a situation of potentially disastrously low oxygen levels . Fish then die from suffocation.

Measuring carbon dioxide is possible using a test kit but it is not normally required to do this.

pH and carbon dioxide

Carbon dioxide dissolves in water to form carbonic acid which has a pH of less than 7 so pH will tend to fall when carbon dioxide is high. This is what happens during the night with a reversal during daylight hours.

There is some quite complex chemistry involved here which I will ignore save to point out that when pond water pH levels fall below 7 then the potential for carbon dioxide problems increases. Pond pH should ideally be around 7.5 to 8.

Aeration of ponds

Aeration of pond water achieves two things both of which are very good for pond water and the fish:

Oxygen levels increase

Carbon dioxide is "blown" out of the water and this tends to push up pH levels.

Remember what Nigel Caddock is very fond of saying. There should only be 3 things in a pond water, fish and air.

Aeration also protects against those algae blooms and their dying - when they die they rot and release carbon dioxide by using up the oxygen resource in the water.

In summary it is difficult to over-aerate a pond and aeration has all round advantages in a pond. The downside is the cost of a special aerating pump. All top koi keepers' ponds bubble with air as do their filters.

Deeper ponds without waterfalls and/or fountains as the means of creating circulation or mixing during calm periods also could be more prone to carbon dioxide problems.

Aeration and water mixing (waterfalls) are the MOST effective methods of controlling potential carbon dioxide problems.

Conclusion:

Do not get carried away but DO beware of algae blooms (green or brown cloudy water) especially in summer and especially during calm periods and when there is no waterfall, fountain or aeration.

Remember the best way to prevent algae blooms is by UV light.