There are lots of diseases that may affect pond fish. The key with any disease outbreak is correct and early diagnosis and the swift administration of the appropriate remedy. Even just a day or two of an undiagnosed outbreak can be disastrous, particularly with the more virulent strains of whitespot seen in recent years.
Fortunately there are lots of very good treatments on the market, stocked at most aquatic outlets. Parasites and fungal/bacterial infections being the most commonly seen.
The most commonly seen parasite is white spot. A combination of factors is usually to blame, stress on the fish from extremes in temperature variation, rough handling/transportation, bad or quickly fluctuating water conditions or other disease organisms. Too much stress from whatever cause can overwhelm a fishes immune system making it weak and prone to infection. Observe your fish carefully when feeding, fishes flicking themselves against the sides of the pond or on plant baskets, especially newly introduced fish should be checked more closely. Whits spots about the size of pinheads may be seen on the fins and body. The whole pond needs to be treated usually over a period of days to break the parasites life cycle. After a whitespot outbreak keep an eye out for secondary infections of fungus or bacteria.
Velvet disease is seen less commonly and may cause the flicking, but looks like a dusting od very fine coppery granules, smaller than whitespots. Its very infectious and is usually caused by overcrowding/bad water quality, so is most likely to occur on newly purchased fish. Fortunately its easy to treat.
The larger parasites, leeches, flukes and worms are less common and mostly can be seen by eye without even catching the fish and can be treated easily once they have been identified. You will no doubt come across them sooner or later in your pond, finding there way as eggs on larvae on birds feet! Before using ANY treatment I would recommend checking your PH, NH3 and NO2 levels. Do not treat if there is any NH3 or NO2, sort out your water quality first. Your PH needs to be above 6.5, below this treatments are less effective and in a badly buffered pond the PH level could crash, this would be a complete disaster. Also bear in mind that the effectiveness of most fish medications is reduced at temperatures below 50 farenheit.
Fungus and Bacteria
Although they are very different organisms fungus and bacteria are often treated with the same medications and are known as secondary infections. They are most likely after a fish has suffered from a parasitic infection that has caused open wounds on the body, or from bad handling that has removed the protective mucous layer from the fishes skin. They can manifest as grey fluffy growths, grey or red spots, the fins being eaten away and looking ragged. Also look out for thick whitish mucous, cloudy eyes and open sores that look raw and ulcerous. Fungus and bacteria can be a lot harder than parasites to clear from a pond and the fish often need a long time to recuperate. Dont forget that all fish medications deoxygenate the water to a certain degree so make sure to have your fountain/watercourse running and/or an airpump running to keep the pond surface active. Especially on warm summer evenings.