How Do You Medicate Fish?



This usually involves the application of a paste/gel onto an external lesion (e.g. applying dilute Betadine over a bacterial skin infection).

PARENTERAL (i.e. via injection)

Intramuscular (IM)

This is the most commonly used method for antibiotic injection. Aim for the muscle block just ventral and caudal to the dorsal fin. The needle should be positioned between the scales and aimed to come in from a caudo-rostral angle. Inject slowly to prevent reflux. The medication is rapidly dispersed by the rich blood supply. This technique is recommended for gravid females and not recommended for small fish and fish with little muscle bulk. The disadvantage of this method is that the injection site may discolour (aim for dark coloured areas on the fish so that melanisation will not show).

Intraperitoneal (IP)

Place the fish on its side or on its back to allow the peritoneal organs to gravitate away from the body wall. Insert needle into this space at a shallow angle to avoid penetrating the internal organs. This is a common route of administrating vaccines as it is done on small fish and will not harm the flesh. It is also good for administrating “painful” medications such as enrofloxacin. The disadvantages of this method are:

• damaging the peritoneal organs or,

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

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

Intravenous (IV)

Fish must be anaesthetised. Insert the needle midline and at 90 to 45 degrees, needle pointing towards the front, just caudal to the ventral fin. Stop just short of the spine. This technique is used more commonly for drawing blood than for administering drugs.


Per os

This is a common delivery method for antibiotics on large farms for ease of medicating large numbers of fish and also minimises handling stress. However, the delivery of the correct doses to each individual fish is difficult (sick fish are usually inappetent and it is these that should actually be receiving the medication). Medication is either sprayed onto the food, impregnated into the food, or prepared with food coated with medicated gelatin, agar or oil. Depending on the drug, this medicated food may sometimes be less palatable. It is thus recommended to reduce the amount of food fed per day by 25-50%.

Gastric intubation

If fish are not eating, they can be anaesthetized and force-fed via a stomach tube. Use a 3mm outer diameter catheter and a 5ml syringe (for larger fish, use a 6mm outer diameter with a 20ml syringe). Administer 1.0-1.15 ml/kg body weight. Due to the high pressure needed to deliver the suspension, the tube may need to be glued to the syringe with cyanoacrylate (e.g. super glue).



This is useful for external parasitic infections and involves using relatively high concentrations of drugs dissolved in the water of a treatment tank. Fish are held in nets and briefly immersed in the treatment solution and then placed into a hospital/quarantine tank. This is often used for new arrivals. Make sure dips are refreshed regularly.


This is a variation of the ‘dip’ treatment. It uses lower drug concentration and involves allowing the sick fish to swim in the medicated water for a period ranging between 2-60 minutes. These are useful for external infections including protozoal, bacterial and fungal. Commercially, this method is used for vaccinating fish against bacterial infection.

Prolonged immersion / permanent bath

This is yet another variation of medicated water where very low concentrations of the drug is used, but the length of exposure is increased. This is particularly useful if the parasites have stages in their lifecycle when they are refractory to treatment (e.g. white spot disease) – the longer exposure time in the medicated water will increase the chances of killing the parasites when they reach the susceptible stage. This method is also used to reduce osmoregulatory stress, when 2-5mg/L of salt (sodium chloride) is added, in conjunction with other forms of treatment. It is also the most frequently used method where client compliance may be low as it is the easiest option.

This article taken from: Loh & Landos (2011) Fish Vetting Essentials. Richmond Loh Publishing, Perth.