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Notes on Specific Antibiotics
The following are some notes on specific antibiotics used in the ornamental fish trade. Many of them are strictly forbidden for use by food fish producers or are otherwise of concern to the Food and Drug Administration (FDA). The FDA considers the use of antibiotics in ornamental fish a 'low regulatory priority.' Officially there are no FDA-approved antibiotics for treating ornamental fish. Therefore, should you have any questions regarding the legalities of using any antibiotics, consult a fish health specialist. For information on antibiotic use in channel catfish, including specifics on use of oxytetracycline (Terramycin®) and Romet B®, see UF/IFAS Fact Sheet VM-70 Use of Medicated Feed in Channel Catfish.
- Erythromycin is most effective against gram-positive bacteria, such as Streptococcus species. The vast majority of bacteria that cause disease in fish are gram-negative, so erythromycin should only be used after culture and sensitivity test results confirm its use. Also, erythromycin is not very effective in a bath treatment and should only be administered by injection or mixed in to the feed. Erythromycin is not FDA-approved for use with food fish.
- The penicillins, including penicillin and ampicillin, are most effective against gram-positive bacteria such as Streptococcus species, therefore, for the same reasons as those given above, these antibiotics are not a good first choice for most bacterial infections in fish. None of the penicillins are FDA-approved for use in food fish.
- Tetracycline and related antibiotics are considered broad-spectrum antibiotics (effective against a wide variety of bacteria) and they work well when mixed with food. (See UF/IFAS Fact Sheet VM-70 Use of Medicated Feed in Channel Catfish.) However, bath treatments may not be as effective for all species. One study (Nusbaum and Shotts, 1981) demonstrated that, in channel catfish, approximately 15-17% of the water concentrations of oxytetracycline was absorbed in water of hardness 20 mg/L and pH of 6.7. However, at least two freshwater fish species (yellow perch and hybrid tilapia) did not have the expected significant levels of this antibiotic in their blood (K. Hughes, unpublished data; S.A. Smith, Virginia Tech, pers. comm. 2002; and R.P.E. Yanong, University of Florida, unpublished data) when exposed experimentally to oxytetracycline as a bath treatment for up to 8 hours. In addition, calcium and magnesium bind to tetracycline and oxytetracycline rendering them inactive. This means that with increasing water hardness (i.e., increases in calcium and magnesium levels) it is necessary to increase the dosages required for these drugs to have any effectiveness as bath treatments. Tetracyclines are ineffective when used as a bath treatment for saltwater fish.
- Tetracyclines are light sensitive, and they turn brown when decomposing. This contributes to poor water quality and may be harmful to the fish. Water should be changed immediately after the bath treatment period is concluded. Due to years of misuse, several bacteria in many different facilities are now resistant to tetracycline. Tetracycline still works adequately against most cases of Flavobacterium columnare (columnaris disease).
- Terramycin is a brand of oxytetracycline manufactured by Pfizer that is FDA approved for use in the production of salmonids, channel catfish and lobsters. (See UF/IFAS Fact Sheet VM-70 Use of Medicated Feed in Channel Catfish.)
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