treatment of fish disease is a subject fraught with difficulties and
misunderstandings. This is in part due to the need to oversimplify of
what is a complex subject.
In all probability there are as many fish killed, directly or
indirectly, by ‘medications’ as there are cured. I am sure that we
would all agree that this would be totally unacceptable with any other
companion animal. Just imagine the uproar if cats and dogs routinely
died as a result of treatments for fairly minor ailments!
Clearly, the main reason that other animals fare better is that they
are usually treated by trained professionals, i.e. veterinarians,
whereas fish, by and large, are treated by their owners. There are
broadly three reasons why treatments often fail:
1. An inaccurate diagnosis (or guess) is
made as to the nature of the problem, which in turn results in the wrong
treatment being used. This is often the situation where ‘dartboard’
medication is used, whereby, a guess is made as to the nature of the
disease (usually based solely on clinical signs rather than an
examination) and a succession of different treatments is applied
in the hope that one of them will work. This can often exacerbate an
existing condition or mean that the condition simply gets worse. This
really is a ‘kill or cure’ approach - sadly too often a kill result.
An accurate diagnosis is essential if health problems are to be treated
2. An accurate diagnosis is made but the
selected treatment is not successful: Treatments may be
unsuccessful for a variety of reasons. In some cases the germs and bugs
(pathogens) are resistant to a particular treatment. It is not unusual
for a treatment that works in one pond to be less successful against the
same parasite in another pond. In such a situation another appropriate
treatment should be used.
Another common problem that affects some disease treatments are
variations in water chemistry. In these situations, water parameters
such as hardness, pH or organic load can interfere with the action of
the treatment, rendering it ineffective. There are also situations,
particularly with severe parasite infestations, where, because of the
host’s reactions to their presence, the parasites are protected by
3. A straightforward treatment overdose:
This can arise from a simple miscalculation of dosages, or by the
toxicity of the treatment being affected by water conditions such as pH
A fourth reason stems, I believe, from a misunderstanding of the
nature of many treatments used to combat disease problems. We can divide
typical treatments into two types. First are the ‘proper’
medications that have been formulated specifically to fight known
pathogens. These would include antibiotics and other types of veterinary
drugs and medicines. These tend to target the disease-causing organism,
and while there may be some side effects, are unlikely to directly
harmful to the patient at the correct doses.
The other class of treatments, and those most likely to be used by
the hobbyist, are not medications in the accepted sense, but chemicals
taken directly from the chemistry lab!
Their action is usually far less discriminatory. They are usually
toxic to all life-forms at relatively low doses. The effective dose is
one that is high enough to kill the smaller organisms such as parasites,
but not high enough to kill larger animals such as fish. The safety
margin is usually very fine. It is important for hobbyists to appreciate
that the majority, including most proprietary treatments, work in this
fashion and that such treatments should not be used in a cavalier
A further consideration are variations in water chemistry. Most
chemicals, with the exception of salt, are affected to some degree by
variances in water hardness, pH and temperature. Many chemicals also
react with dissolved and particulate organic matter such as fish waste,
algae and detritus - thus affecting or reducing their efficacy.
The three most important steps are 1) an accurate diagnosis.2)
an accurate dosage calculation and 3) a follow-up examination to monitor
the effectiveness of the treatment.
When applying any treatments
Make sure that the water is well
oxygenated as many treatments can remove oxygen from the water. This
is particularly important with any formalin-based treatments.
Turn off any UV lights as the
ultraviolet radiation may degrade or affect the chemical.
Consider whether it is necessary to by
pass the filter system to avoid adversely affecting the filter
biomass. This would in turn result in a loss of water quality, which
would simply make the situation worse.
Keep a close eye on fish under
treatment as even at the correct doses some may react badly. Always
be prepared to terminate a treatment under such circumstances.
Consideration should be made as to how a treatment could be rapidly
diluted if needed.
Many treatments are degraded in strong
sunlight, particularly potassium permanganate.
Consider netting the pond as fish may
well jump out when treatments are applied.
i.e. 5 ppm = 5 mg / litre
mg / litre x
3.785 = mg / gall
i.e 5 mg / litre = 18.9 mg / gall (US)
mg/ litre x 4.546 =
mg / gall
i.e 5 mg / litre = 22.7 mg / gall (UK)
To convert imperial
gallons to US gallons multiply by 1.2
1 ounce = 28.35 grams
1% solution =
10 ml per litre
10 gram per litre
38 gram per
45 gram per gall