This usually involves applying a paste / gel to an external lesion (for example, applying diluted Betadine to a bacterial skin infection).

Dad (i.e. by injection)

Intramuscular (IM)

This is the most widely used method for injecting antibiotics. Aim for the muscle block just ventral and caudal to the dorsal fin. The needle should be placed between the scales and pointed inward from a caudo-rostral angle. Inject slowly to prevent reflux. The drug disperses rapidly through the abundant blood supply. This technique is recommended for pregnant females and not for small fish and fish with little muscle mass. The downside to this method is that the injection site can become discolored (aim for dark colored areas on the fish so that melanization is not seen).

Intraperitoneal (IP)

Lay the fish on its side or on its back to allow the peritoneal organs to move away from the body wall. Insert the needle into this space at a shallow angle to avoid penetrating internal organs. This is a common route of vaccine administration, as it is done in small fish and does not harm the meat. It is also good for administering “painful” medications such as enrofloxacin. The disadvantages of this method are:

• damage the peritoneal organs or,

• if injected into the ovary, the drug will not be redistributed to the rest of the body and

• if injected into the intestinal lumen, the drug will be excreted.

Intravenous (IV)

Fish must be anesthetized. Insert the needle midline and 90 to 45 degrees, with the needle pointing straight ahead, just caudal to the ventral fin. Stop just before the spine. This technique is more commonly used to draw blood than to administer medications.


Per os

This is a common method of administering antibiotics on large farms to facilitate the medication of large numbers of fish and also minimize handling stress. However, delivering the correct doses to each individual fish is difficult (sick fish are generally not appetizing and it is these that really should be receiving the drug). Medication is sprayed on food, soaked in food, or prepared with food coated with medicinal gelatin, agar, or oil. Depending on the medication, this medicated food can sometimes be less palatable. Therefore, it is recommended to reduce the amount of food that is administered per day by 25-50%.

Gastric intubation

If the fish do not eat, they can be anesthetized and force-fed through a stomach tube. Use a 3mm OD catheter and 5ml syringe (for larger fish use 6mm OD with a 20ml syringe). Administer 1.0-1.15 ml / kg of body weight. Due to the high pressure required to deliver the suspension, it may be necessary to glue the tube to the syringe with cyanoacrylate (eg super glue).



This is useful for external parasitic infections and involves the use of relatively high concentrations of drugs dissolved in the water of a treatment tank. The fish are kept in nets and briefly dipped in the treatment solution and then placed in a hospital / quarantine tank. This is often used for newcomers. Make sure the sauces are updated regularly.


This is a variation of the “dip” treatment. It uses a lower drug concentration and involves allowing sick fish to swim in the medicated water for a period of between 2 and 60 minutes. These are useful for external infections that include protozoa, bacteria, and fungi. Commercially, this method is used to vaccinate fish against bacterial infections.

Prolonged immersion / permanent bath

This is yet another variation of medicated water in which very low concentrations of the drug are used, but increases the duration of exposure. This is particularly useful if the parasites have stages in their life cycle when they are refractory to treatment (eg white spot disease); longer exposure time in medicated water will increase the chances of killing parasites when they reach the susceptible stage. This method is also used to reduce osmoregulatory stress, when 2-5 mg / L of salt (sodium chloride) is added, along with other forms of treatment. It is also the most widely used method when customer compliance can be low, as it is the easiest option.

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