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Seasonal Impacts Upon Carbon Dioxide Pond Problems, Aeration & pH & Blanketweed

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 (algae) and animal life has also slowed down significantly.

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 more. The living organisms are therefore emitting more carbon dioxide in a situation of potentially disastrously low oxygen levels. Fish can die from suffocation under these conditions.

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

pH and carbon dioxide in pond water

Carbon dioxide dissolves in water to form carbonic acid that 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 we 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 to get dissolved oxygen concentrations higher

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.

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 and especially one with algae. 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.

Dissolved oxygen is also critical in solving algae problems the natural way using barley straw as will be discussed later.

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.

Non-Circulating Pond Surface Algae

Algae that grow to about 1 cm in length and are seen on pump surfaces, rocks, pump filter sponges and the sides of ponds are normal. Do not try to remove them by sweeping the pond walls for example because they will keep coming back and if left alone reach an equilibrium point. The fish love them and the balance of the small pond is improved.

They can be a bit unsightly sometimes but this is natural. They will not damage your pond environment. UV light will not affect these algae in any way because they stay in one position and are NOT exposed to the UV light.

Dreaded Blanketweed ... The enigma of blanketweed.

Blanket weed is that long slimy algae that seems to grow and grow and can never be eradicated.

It consists of long thin strands which as you can see are made up of single cells in a straight line. These strings grow like crazy.

Many pond keepers never see it and others are never without it. It is an area of pond keeping not well understood and many of the suggested solutions do not always work. UV lights are no good because the algae are not exposed to the light - it remains in the pond.

Often even when there is lots of blanketweed the water is crystal clear. This is because the blanketweed is starving other algae out of existence, helped by reducing light penetration through the floating blanket

By improving the balance between plants in the pond and reducing fish density and therefore reduced feeding the tendency to get blanketweed will be reduced but not necessarily eradicated. Plants in ponds compete with the algae for the "fertilisers" produced by the fish as explained earlier.

Water quality certainly plays a role in blanketweed development but this is not the whole story since even the best pond keepers can get this problem. We do have a good solution so long as you follow the instructions below. It involves barley straw and while many people have heard of this treatment not many understand the circumstances under which it works. Failure to understand may well result not only in failure to control blanketweed but also in fact to create other pond problems.

Be careful when buying chemicals to kill blanketweed - make sure they do not kill anything else including plants, fish, filter bacteria, birds that drink the water etc. At Bradshaws we have an excellent solution to blanket weed called "Goodbye Blanket Weed"


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