Pond problem solver – gasping fish
All pond fish breathe by pumping water over their gills to extract oxygen, which is dissolved in the pond water. Fortunately, compared to other fish species, the carp family do not require an especially high level of dissolved oxygen (DO) which makes them relatively undemanding as ornamental pondfish. Even when, through whatever reason, DO does fall below a pondfish’s minimum requirement, they can adapt by coming to the surface to gulp in air as a short-term survival strategy. However, this is extremely stressful for fish and if you witness such an extreme change then immediate action is required.
As is the case when confronted by any pond fish-related problem, a responsible pond keeper should approach it in 2 ways and ask a couple of questions.
1.How can I remedy the problem and avoid stressing the fish any further?
2.What can I learn from this experience as far as what caused it and how can I prevent it
from happening again?
A gasping response in pond fish will not necessarily always be caused by an unhealthy drop in DO but may also be behaviour shown by fish that is not getting sufficient oxygen to its tissues for another reason.
Besides a drop in DO (which is the most likely cause of gasping behaviour) another cause could be that the koi are suffering from poor water quality, specifically nitrite toxicity.
If for some reason, toxic nitrite is not being broken down and detoxified effectively by a biofilter, then it will cause a build up of nitrite within a fish’s blood and tissues. When nitrite reacts with the oxygen-carrying haemoglobin within the blood, it forms a stable non-oxygen carrying form of haemoglobin, which will in turn cause the fish to gasp. A simple water test for nitrite will confirm or deny its presence.
For this reason, if the cause of gasping in pondfish is a nitrite reading, in addition to adding extra aeration it would be wise to carry out a water change to dilute the toxic nitrite.
Other more common causes of gasping in fish can usually quite simply be identified and easily rectified. These include:
1. Hot Weather
Yes, occasionally we can suffer from hot weather in the UK. As water warms up, its ability to hold DO decreases and yet a fish’s oxygen requirements increase. The DO requirements of other organisms in the pond such as bacteria and other aquatic life also increase.
If a pond is running close to a comfortable oxygen level prior to such a hot spell then when the water temperature rises, the ‘double whammy’ effect described above can leave fish and the pond environment oxygen-deficient.
A well-planted pond in full sunlight is a real asset as a natural source of oxygen as the plants produce far more oxygen in photosynthesis than they use in respiration. However, at night, the same plants will still be using oxygen but will not be releasing any oxygen through photosynthesis. This will cause a drop in pond DO causing the fish to gasp, particularly in the early morning as a result of a phenomenon called dawn depletion. A remedy for this is to thin out excessive submerged plant growth and improve aeration or circulation within the pond.
Some pond medications when added to the water can cause quite a severe drop in DO through chemical reaction. Medication may also reduce DO through adverse interaction with other pond life. Extra aeration should be added during any course of pond treatment as a precaution and extra care should be taken after the use of an algicide. Dead algal matter will put an extra burden on the oxygen balance within the pond as bacteria break it down.
3. Blocked filter/pump
This cause of low DO may sound obvious but a blocked pump or filter may cause problems by stealth where a gradual, almost unnoticeable drop in turnover occurs. This will have an adverse knock-on effect for DO levels.
Regular flushing and maintenance of filter media and pump pre-filters will keep this from becoming a real problem. Blockages are more likely to be an issue in the warmer months when excessive algae growth is experienced.
It is reassuring to know that a pond can not be over-aerated and through the many different means and devices available today, barring accidents, low DO should not be a problem in a well managed garden pond.
Filtration will nearly always benefit from the addition of extra aeration, which in turn will lead to better water quality. Venturis, air stones, micro-bubblers, and even air domes fitted to bottom drains allow easy, effective and efficient aeration and are a must for any serious pond keeper, especially when stocking levels are high.