Treatment of Fish

There are three basic techniques for treating fish.

1.   Adding drugs to the water (Water-borne drugs): Drugs may be added to the water in three different ways.

a.     Dip method: In this technique, a concentrated chemical solution is made up in tank. Fishes or eggs are placed in hand net or trays and dipped into solution for few seconds. This method is used for dis-infection of raceways and aquaria for control of the disease.

b.     Bath method: The most common method of administrations therapeutic agents to fish is bathing in water-soluble compounds. This may carried out in ponds or tanks. Where the water is static during treatment and the fish are left in solution for certain period. Fish suffer from many external diseases (parasitic, fungal and bacterial) can be treated by this method. There are three types from bath treatments. The difference between these bath treatments is the concentration of the chemical applied and the period of time that the fish are in contract with chemical.

i.    Dip bath: The fish is dipped into a concentrated chemical bath for a short period of time, often less than one minute. Prolonged exposure to the chemical at the high concentration delivered in a dip would be fatal to the fish. Because of fatalities can easily result from an improperly administered dip treatment, and because diseased fish are generally intolerant of stressful situations a short period. This method has the advantage of handling large numbers of fish in a short time.

ii.    Short bath: Short bath means that the fish are subjected to a moderate chemical concentration for a period of time ranging from 30 minutes to several hours. This is an excellent method for administering many medications to fish kept in aquaria, tanks, or raceways. The duration of exposure to the chemical will be determined by chemical used, the concentration of chemical used and the facility in which the fish are housed. In most cases, water flow and filtration are shut down while the chemical is in contact with fish. It is widely used in treatment of fish (parasitic diseases).

iii.    Prolonged bath: It means that small concentrations of chemical are applied and left in the water on a permanent basis. This is the only method of administering a bath to pond fish. Because of the low concentrations of chemical applied, a prolonged bath is often the least expensive and safest way of administering a chemical bath.   May last for many hours to days. It is therefore possible only when the therapeutic index of drugs is large.

c.      Flush method: Flush is a modification of the bath treatment for flow through system. Water flow is not stopped, but a high concentration of chemical is added at the input and passed through the system as pulse. The introduced dose should be added in 1 to 2 minutes. A measured amount of drugs is added to the system upstream and allowed to flush through. Flush has been widely used in salmonid hatcheries. Flush treatment is only suitable for systems that have enough flow to completely flush out the drug within a predator minded time.

2.   Adding drugs to the feed: Medicated feeds are widely used to administer drugs to fish for systemic infections.

         3.  Administration of drugs to individual fish: Valuable fish can be treated individually by local application or by injection (s/c, I/m and I/p).


Substances Used In Treatment

1.     Salt (NaCl): Salt is one of the most commonly used drugs in aquaculture. In fact, it is sometimes referred to as the aspirin of aquaculture. Many forms of salt are used including table, meat-curing, rock salt Sodium chloride can treat many external parasites and also used to relieve stress during handling and transport. The methods of application, concentration and duration of application depends on the disease, species, weight and type of aquaculture unit.

2.     Formalin: Formalin is used as a bath treatment to control external parasitic infections of fish .It is extremely effective against most protozoans, as well as some of the larger parasites such as monogenetic trematodes. Formalin kills parasites on gills, skin, and fins. It is not the preferred treatment for external bacterial or fungal infections. In addition, high concentrations of formalin are used to control fungi on fish eggs. In ponds by prolonged bath method, a dose of 15 to 25 mg/L (2 drops /gallon) may be used. By short-term baths: dose 250 mg/L for 30-60 minutes. When the water temperature increased the concentrations of formalin should be decreased.

3.     Malachite green: Used for treatment of many external protozoan and fungal infections. Concentration 0.1 ppm in pond or aquaria at three or four days intervals. It’s may be administered as bath treatment at 2 ppm for one hour.

4.     Acriflavine: Used in the treatment of fish affected by protozoa, skin and gills flukes, fin rot and fungal condition. The concentration 0.02 mg/liter for three days and repeated if required. Disadvantage of acriflavine, insoluble in salt water and phytotoxic.

5.     Potassium permanganate: Correct use of potassium permanganate can effectively control many bacterial, parasitic and fungal agents before systemic infections become established For most fish, potassium permanganate can administered at a concentration of 2mg/L as a long- term bath (4-hour minimum) in fresh water or salt water system.

6.     Antibiotics: Antibiotics are used extensively in fish culture in treatment of problems caused by bacteria. Antibiotics doses for 5 to 10 days are found in the following table:






3.5 gr. /100 1b fish /day


4-8 mg/L

4.5 gr./1001b fish/day


10 mg/L

4 gr. /100 1b fish /day


0.025 mg/L



8000-12000 U.I/L

2.5 gr./100 1b fish/day

The sensitivity test must be applied to determine the effective antibiotics against the causative agent. The dose depends on nature of the disease, types of antibiotic, age & weight of the fish.


Precautions Taken During Treatment of Fish Through Water

1.     Do not feed the fish for 24 hours before treatment.

2.     Using plastic buckets for mixing the drugs.

3.     Accurate calculations of the dosage. This based on rates of water flow.

4.     Apply treatment during the time of the day when the temperature is lowest.

5.     Carry out a trial for treatment on few fish as control before making the treatment on large scale.

6.     Watch the fish continuously during treatment.

7.     Only repeat the treatment if absolutely necessary but not within 30 hours of the first treatment.

Principles of Prophylaxis

There are two ways for prophylaxis of fish from infection.

1.     Protection:

a.     Provision of pathogen free water by:

i.    Filtration: Prevent the entry of infective stage of larvae but inadequate to prevent the entrance of microorganisms.

ii.    Ultraviolet irradiation: The best effective method against micro-organisms in water but cannot applied on large scale.

iii.    Chemical treatment: Produced unsuitable environment & need experience to apply.

b.     Provision of pathogen free food.

c.      Hygiene of the fish farm:

i.    Disinfection of the habitat by:

a)     Disinfection of fish farms every 3-4 year Drying and added quick lime.

b)    Dead fish must be removed.

c)     Aquatic vegetation must be controlled.

ii.    Disinfection of the equipment’s and utensils to prevent spread of infection from one farm to another.

iii.    Disinfection of fish twice yearly by:

a)     Sodium chloride

b)    Malachite green

c)     Potassium permanganat

d.     Control of the wild fish.

e.      Vector must be controlled (snails and birds).

f.       Quarantine: Fish which are moved form suspected area or infected area to non-infected area must be help in detention for a period of time at least as long as the incubation period of the disease.

g.     Regular prophylactic survey.

h.     Random samples must be taken from fish farm for different examination to ensure that the farms free from infectious diseases.

i.       Independent water supply.

j.       Age segregation.

2.     Prevention:

a.     Water: fish need water that is not only pathogen free but also meets the species-specific requirements of temperature, oxygen content & purity.

b.     Food: fish must be provided with right kind of food with sufficient quantity. Nutrition deficiencies produced disease & also make the fish less able to resist the pathogen.

c.      Vaccination: vaccination can be by injection, immersion, bath, spray & the oral route. The efficacy of vaccination is influenced by the quality of the vaccine, the ability of the fish to develop immunity and the conditions under which the fish are vaccinated. In produces very good protection against some diseases. Vaccination should be carried out under the following rules:

i.    Do not vaccination diseased fish.

ii.    Starve the fish for 48 hours before vaccination.

iii.    Vaccine must be applied under aseptically condition as possible.

iv.    When using the injection method, be sure to apply the vaccine correctly.

v.    Do not vaccinate fish during smoltification.

vi.    Vaccinate small number of fish & observe for a week before the rest of the population.

d.     Genetic manipulation to produce species that are resistant to disease.

e.      Stress factors must be removed or avoided.

f.       Population density: Accurate calculation of the number of fish /unit is necessary. Suitable number of fish should be put in suitable unit to ovoid the stress factors. Overstocking increase competition among individual fish (10.000 to 50.000 fry per hectare).