Froth or foam appearing on the surface of pond water will generally sound alarm bells in the heads of most fish keepers, and rightly so. Its unsightly, man-made quality is a clear sign that there is a water quality problem; even though the main body of water can remain clear and the fish appear perfectly healthy.

Even so, frothy or foamy water should always be regarded as a warning sign which – if left unresolved - can lead to a worsening water conditions which will eventually result in health problems for your fish!

Most ponds with a waterfall or fountain have some foam or froth from normal water agitation and this is perfectly natural. However, as soon as the foam begins to accumulate and spread across your pond, it is time to take action.

There are a number of reasons why this foam can form, but the most common is due to an excess of dissolved organic compounds and notably phosphates. These compounds can arrive from a number of sources including overfeeding, a build-up of fish waste, or decaying plant material. They are also likely to indicate rising levels of ammonia, nitrates and nitrates which at critical levels can become deadly to your fish.
Phosphates occur naturally in living and decaying plant and animal remains, and as mineralized compounds in soil, rocks, and sediments. Within a pond the most prominent sources of phosphates will be uneaten fish food, decomposing fish, animals and their feces. In small amounts, phosphorous may not seem a bad thing as it can produce a boost in plankton and algae, enabling fish to grow larger and faster. However, in larger amounts, phosphorous can make aquatic systems so productive that they can choke themselves out!

While rising levels of phosphates may not be an immediate and serious problem in itself, they are usually closely linked to organic compounds which are. The clue is in it how the phosphates have entered the water which - as mentioned before – is through the breakdown of uneaten fish food, decomposing fish, animals and their feces. This decomposition links directly to the nitrogen cycle where ammonia, nitrite and nitrates are produced, all of which can have a toxic effect on your fish.
Ammonia is extremely toxic and even relatively low levels pose a threat to fish health. Ammonia is produced directly from the fish via its gills, decomposing fish food, fish waste and detritus, but in a natural environment - such as a lake or river - it would be immediately diluted to harmless levels. However, in the confines of a pond, ammonia levels can rapidly rise to dangerous levels unless it is constantly removed, usually by biological filtration.
Nitrite (NO2-) is formed when Nitrosomonas sp. bacteria oxidise ammonia. Although it is less toxic than ammonia, elevated levels will still present a threat to fish health. Prolonged exposure at low levels can lead to stress and is often associated with stress-related disease such as bacterial ulcers and fin-rot. At high levels, skin and gill epithelia can be damaged and opportunistic bacteria and parasites may take advantage of stressed fish. The main danger is from nitrite being actively transported across the gills and into the fish’s bloodstream where it oxidises normal haemoglobin into methemoglobin. Normal haemoglobin picks up oxygen at the gills and transports it to the body tissues where it is exchanged for carbon dioxide. Methemoglobin cannot transport oxygen and therefore - in acute cases - fish will be effectively asphyxiated.
At low levels of Nitrate concentration you may find fish rubbing against solid objects. As levels increase fish will become lethargic, but may still swim up to feed. If the fish is suffering from nitrite poisoning, the gills will change from a healthy pinkish/red to a pale tan to dark brown in colour. The fish may also show signs of respiratory distress, i.e gasping at the water surface or hanging around water inlets.

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