A range of problems
Koi suffer can suffer from a whole range of health problems and
diseases including hypoxia, gas bubble disease, dropsy and general lumps
In the case of 'dropsy' or 'pine-cone disease' the fish's body often
swells up, the eyes may start to bulge out (exophthalmia) and some or
all of the scales may stand out from the body, giving the appearance of
a pine-cone, hence the common name. This is an extremely serious
condition having many possible causes. It is important to understand
that it is merely a clinical description of the condition: there is no
actual disease called ‘dropsy'. Dropsy could indicate a contagious
infection so it makes sense to isolate any affected fish.
A long-term bath of salt at one-ounce per gallon may help reduce
osmotic stress and in general I would recommend a course of antibiotics
in case the problem is bacterial. It is reasonable to assume that the
condition is stressful to fish and, unless there are signs of
improvement, it may be kinder to euthanase the fish, rather than prolong
Hypoxia is 'oxygen starvation'. Most koi ponds are usually well
served with venturi retums or waterfalls, but poor pond maintenance,
high stocking levels and unusual climatic conditions can lead to low
dissolved oxygen (DO) levels. Low DO is likely to occur in summer. As
water becomes warmer it can progressively hold less oxygen: and the fish
become more active, leading to a greater demand for oxygen; and the
bacteria in the pond and filters need more to, as do submerged green
plants including algae.
The role of submerged plants and algae should perhaps be clarified.
During photosynthesis, submerged plants release oxygen into the water,
which is why they are often called oxygenating plants. However, they
also respire at the same time, extracting oxygen from the water and
excreting carbon dioxide. During daytime they produce more oxygen than
they consume, but at night, when photosynthesis ceases, respiration
continues and they become net oxygen consumers.
Clearly, if the oxygen demand exceeds the oxygen supply then the DO
levels will gradually decline and this presents a serious danger to the
koi. Common causes of low DO, apart from high fish densities, are heavy
feeding and a dirty pond or filter.
A lot of oxygen can be used in oxidising organic waste and, under
certain conditions, this extra demand may be 'the straw that breaks the
Unfortunately, a DO problem often occurs in the early hours of the
morning. when we are not there to see its direct effects on the fish,
rather than during the day when submerged plants are releasing oxygen
Typical clinical signs of low DO are lethargy and a tendency for the
fish to gasp at the water surface and congregate around water returns.
Many of these signs are the same as for a gill problem so a test for DO
has to be made to be conclusive. There are cheap DO test-kits available
but it is important to follow the instructions carefully to avoid
introducing oxygen into the water sample by agitation or by pouring
water into the test phial. If low DO is the problem, i.e. DO
is less than 5 to 6 ppm (mg/litre), then additional aeration will help -
but it is essential to determine what caused the problem and to take
'Gas bubble' disease
'Gas-bubble' disease (which is uncommon) presents the opposite
problem to hypoxia. It is caused by over-saturation of the water with
air. During summer it is tempting to try and force as much oxygen as
possible into the water but it is important to realise that at any given
temperature there is a maximum amount of oxygen the water can naturally
hold If this level is exceeded for example by forcing air into the water
under pressure the water can become supersaturated with oxygen.
Given my earlier comments about the dangers of hypoxia, this may seem
a desirable state of affairs but it isn't. If excess air is forced into
water it will gradually come out of solution as the natural equilibrium
is restored. This excess air may come out of solution in the fish's
Typical signs of which are visible air bubbles in the eyes, gills and
sometimes in the skin. This problem can be detected by measuring total
dissolved gases (DO, nitrogen, argon, dissolved carbon dioxide etc.)
though the equipment required is expensive.
Lumps and bumps
Koi are often seen with swellings and growths. The causes and
conditions can be complex and I am in danger of making oversimplified
comments on what is a specialist area of fish health. It is worth trying
to make a few generalizations but, if your fish are affected, seek
If a fish has any unusual lumps try to determine whether they appear
either over or under the skin. Often it is easy to decide but there are
some skin conditions which result in very large tumours, where there
tends to be very little distortion of the surrounding, unaffected,
tissues. Deep-seated (below the skin) lumps tend to distort and distend
a larger area of seemingly healthy tissue.
Where a deep-seated lump or growth is suspected, there are several
possible causes; for example, an internal tumour or the onset of a
bacterial problem. In this case expert opinion should be sought, as any
delay could be serious.
Most skin growths, while disfiguring, are rarely fatal. These can be
small milky white-to-grey growths that resemble drops of candle-wax,
typical of carp pox. Such growths are often seen on the fins, especially
during early spring. At the other extreme, some skin tumours can grow to
the size of tennis-balls and range in colour from white to pinky-red.
While they can be benign, expert opinion is advisable, especially if the
growth interferes with normal swimming and feeding.
In general, there is little that can be done about growths, as many
are caused by viruses or other unknown factors. It is possible to
attempt surgical removal of larger growths, but this is not usually
advisable unless there is a compelling reason to do so. With any type of
growth, there is always the risk of secondary bacterial infection so the
condition needs to be monitored closely. Often, the problem tends to
resolve itself, though it may take a while and, as with any viral
problem, there is a strong possibility of a recurrence.