An Underestimated Problem
High concentrations of carbon dioxide (CO₂) prevent oxygen absorption. The fish become nervous, reel and in a later stage are practically drugged.
Higher levels of hardness are less critical due to the ability to bind acids.
Older ponds with soft water that are diluted with a lot of rain can become problematic.

The Problem with Older Ponds
Carbon dioxide forms through the decomposition of sediments on the ground.
Problems only really appear in older ponds after longer periods of frost. It is easy, however, to clear small surfaces from ice and 'skim off' the excess CO₂.
Then again, many ponds have enough sediment in the first winter to become critical with three months of ice on the water.

Too Little is Often the Problem Many ponds are impaired by a deficit of CO₂. Intensive water movement and aeration drives CO₂ out of the water, so that there is virtually none left. Plants, too, can use up practically all CO₂.
When the concentration drops below 1mg/l breathing is severely hampered and a characteristic swelling of the gills occurs.