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:
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
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
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
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
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
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