Page 30 - BKKS National Show Programme 2018 highres_Neat

Blowing bubbles

underwater –

Aeration explained

by Syd Mitchell

The air we breathe contains about 21% with temperature which increases their oxygen demand.
oxygen which is equivalent to 210 mg/L. In There is both an optimum and maximum temperature
summer, koi are lucky if their pond oxygen at which koi can live and function. At the optimum
level is 10mg/L which gives them only 5% temperature, oxygen consumption is high because of rapid
of the oxygen available to us. Worse still, growth and significant activity. Above this temperature,
koi begin to experience stress. Stress triggers their
because water is much more viscous than air, warning and defence systems which require very high
it takes energy to push water through their oxygen consumption. Unfortunately, the amount of oxygen
gills which means that 10% of the oxygen koi available in the water also decreases with temperature.
can extract is actually used to power their The combination of these two events limits the maximum
respiration cycle so how do they manage to temperature at which the koi can survive.
stay alive and active? Minimum oxygen concentration
Concentrations of dissolved gasses in water are much lower This depends on activity, long term acclimation and stress.
than in air. A typical pond at a temperature of 20°C will have Young koi are less tolerant of low oxygen levels than older
dissolved gas concentrations of about 14 mg/L nitrogen, 9.1 ones even though larger koi consume much more oxygen.
mg/L oxygen, and 0.5 mg/L carbon dioxide. As these gases Oxygen levels between 3-5 mg/L can normally be tolerated
dissolve into the water, a point is reached where no more for a few hours but, if levels fall below 3 mg/L, fish will be
can be added. This is called saturation. Saturation points gasping for air at the surface or crowding around air stones

Saturation and 80% saturation values of oxygenin fresh water at sea level
Temperature 0°C 5°C 10°C 15°C 20°C 25°C 20°C
Oxygen saturation mg/L 14.6 12.8 11.3 10.1 9.1 8.3 7.6

80% saturation mg/L 11.7 10.2 9.0 8.1 7.3 6.6 6.1

are different for each gas and are dependent on different or points of higher concentration. Death due to hypoxia will
factors, but temperature is the most important. quickly follow.
For ponds at sea level, the dissolved oxygen saturation level
is shown in the table. These values are for fresh water with
no impurities. An increase in altitude above sea level or Ideally, our ponds should be at or near oxygen
saturation. Although koi will tolerate 5-6 mg/L, the
additions to the water such as chemical treatments or salt
will decrease these saturation levels. Salt at 5 grams per minimum acceptable level is 80% saturation and growth
litre (5 ppt) will decrease the dissolved oxygen saturation rates are reduced if the oxygen levels are 25% below
levels by about 1 mg/L. A 1,000 metre (3,000 foot) elevation saturation.
increase above sea level will also decrease the dissolved The efficiency of the biofilter will only be at a maximum
oxygen saturation levels by about 1 mg/L. when the water entering it is near saturation.
Fish are well adapted for extracting oxygen from the very Oxygen tests should be done when the temperature is at
low concentrations found in water. The rate at which oxygen its highest. Although plants and algae add oxygen during
is consumed by koi is related to the water temperature. Koi the day they can dramatically deplete the level during the
are poikilothermic meaning that their body temperature is night so it is sensible to verify that dissolved oxygen is still
essentially that of their environment and their metabolic high first thing in the morning. If oxygen tests are done
activities are enzyme-catalyzed chemical reactions that are occasionally and confirm that at least 80% is being achieved
temperature dependent. Metabolism and activity increase then a simple visual check that air is still pouring into the
30

BKKS National Show Programme 2018.indd 30 07/06/2018 12:38