In every garden pond or even a simple recirculationg water feature you will find life of some kind.
In this article I want to talk about what happens in garden ponds throughout a normal span of 24 hours. If you understand these basics this will go a long way to your getting even more enjoyment out of your pond.
Think of a pond as a collection of living creatures, fish, plants, algae, frogs, bacteria and tiny insects. All of these breathe like you and I and all need oxygen (well almost all). All these flora and fauna exhale carbon dioxide. Think of your pond as breathing-in oxygen during the day, and breathing-out carbon dioxide at night.
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
Of course all of us think first and foremost of oxygen in pond water as being essential and of course this is 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. From dawn to dusk (daytime) oxygen levels in the 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
Dawn is the critical time in a pond. If something is going to go wrong it is often just before first light because at this stage the oxygen level dissolved in the pond water will be at the minimum.
Often people wake up to find dead fish... and ask 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 are more likely to 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 may show very significant variations.
Seasonal Impacts upon oxygen in water.
The first thing to realise is that oxygen concentrations are highest in winter because water is cooler. 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 meaning they need less oxygen.
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.
You can measure amounts of carbon dioxide but it is not recommended.
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 pond pH should ideally be around 7.0 to 8 but don't panic if it is 8.5.
In practice this means that measuring pH can give very erroneous results depending upon time the sample is taken. If you do like to measure water properties then do this on a continuous basis at the same time every day and keep a graph of results so you can monitor changes. Individual results can be quite misleading.
Aeration of ponds
I advise that all ponds should be aerated using either a simple fountain or waterfall at least. Many pondkeepers use an air pump with associated air stones. Another alternative to adding oxygen to pond water is to use a venturi device.
Aeration of pond water achieves two things both of which are very good for pond water and the fish:
1. Oxygen levels increase
2. Carbon dioxide is "blown" out of the water and this tends to push up pH levels.
Remember what Nigel Caddock, one of the UK's top koi keepers, is very fond of saying. There should only be 3 things in a pond: water, fish and air. Add plants to this list.
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 fish keeping. The downside is the cost of a special aerating pump. All top koi 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 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 be over-concerned 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. Do install a fountain or waterfall at the very least if you keep fish in a garden pond.
Remember the best way to prevent algae blooms is by using an UV light.
Tony Roocroft is the author of "The Complete Pondsolver". You can learn a great deal about keeping your fish pond clean and clear all year round by spending a bit of time exploring his website http://www.practical-water-gardens.com
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