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Medicated / antibiotic treated food To treat bacterial disease

When a bacterial disease has been diagnosed, one treatment option is to use antibiotics. There are three main routes for antibiotic usage; baths, injections and medicated food. Using medicated food is an option that minimises handling and therefore stress – although affected fish may still need topical treatment for moderate to severe lesions.

One disadvantage of using medicated food is that it is not possible to ensure that individual fish receive the right dose of medication; indeed sick fish may not feed at all. The other problem with this method is the limited choice of antibiotics in ready prepared food – which for hobbyists in the UK is restricted to oxolinic acid. It is possible to “top dress” ordinary food with other antibiotics but this brings with it the problems of palatability and the risk of antibiotics being washed off the food before it is eaten.

Ready-prepared antibiotic foods

As already mentioned, in the UK oxolinic acid is the only antibiotic treated food available to hobbyists. Unfortunately, because of previous overuse and misuse, there is a very high level of bacterial resistance to oxolinic acid. Other antibiotics are available in commercial fish foods used to treat food fish such as salmon and trout, but my understanding is that these are not available to hobbyists. All ready-prepared medicated food requires a veterinary prescription – which will involve a visit to your local vet.

  • For further details phone: King British Ltd. Mr. Holmes 01274 576241

  • For more details on commercial preparations phone: Vetrapharm Ltd. 01425 656081

  • Commercially available medicated food in America

    Top dressing ordinary food

    The other option is to apply antibiotic directly to ordinary pelleted food. This increases the options of which antibiotic can be used. Again, the antibiotic will have to be obtained via your local veterinary surgery. The two major problems (once you have calculated the dose) are getting the antibiotic to bind to the food and not float off when the pellets are put in the water, and palatability – that is making sure they still taste nice! Tests carried out at the Fish Disease Laboratory in Weymouth showed either vegetable or fish oils to be the best binding agents and to have the least affect on palatability.

    Preparing the food

    Using table 1 calculated the total amount of antibiotic required. First calculate the total weight of the fish to be treated. As a rough guide, a 4” fish will weigh 100 grams. 12” fish will weigh approx. 1kg. 18” fish will weigh approx. 2kg.

    Determine the total amount of antibiotic required per day. For example if the total weight of fish was 30kg and you were using oxytetracycline at a dosage of 75mg/ kg body-weight, you would need 30 x 75mg = 2.25 grams of active ingredient per day.

    On a 14 day course the total amount of active oxytetracycline required would be 14 x 2.25 grams = 31.5 grams active ingredient (you should be aware that not all antibiotics are 100% pure).

    Calculate the amount of food the fish will eat over a 14-day period. A good estimate is 1% total body-weight per day. In our example, 30kg of fish will eat approx. 30kg x 1% x 14 = 4.2 kg food

    Use a small amount of oil; say 2-3 grams per 100grams of food. Warm the oil to about 40oC and quickly stir in the total antibiotic for the 14-day period

    Add the oil and drug slurry to all the pellets and stir to spread the oil slurry evenly over all the pellets. Cool and divide into 14 equal portions. Store in a sealed container in a cool room or refrigerator.

    It is sometimes helpful to starve the fish for 24-hours before starting the treatment

    Table 1: 

    Antibiotics used as top dressings to treat bacterial disease in ornamental fish


    oral: 40-80 mg/kg body-weight daily


    oral: 50-100 mg/kg body-weight daily


    oral: 30 mg/kg body-weight daily


    oral: 10 mg/kg body-weight daily

    oxolinic acid oral: 10 mg/kg body-weight daily

    oral:75 mg/kg body-weight daily


    Using medicated food is not as effective as antibiotic injections. Where just a few fish are affected, injections are the best route to follow. However, where large numbers of fish are affected, or it is not practical to catch the fish on a repeated basis, then medicated food is the next best practical treatment method. 

    Fish disease problems? Need some help? The help pages take you through the basics of the 'fish health work-up' diagnosis method and basic fish disease treatment methods.

    90% of all health problems and fish deaths are caused by poor fish keeping! The key pages are essential reading for all beginners and intermediates who are serious about disease prevention.

    Expanded Content by Dr. Erik Johnson, and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved