The active constituent of Aquari-Cycline “Tetracycline Hydrochloride” is now legal for open sale (S6) for use in ornamental fish. Bacterial diseases can be devastating in aquariums and rank along with other water quality problems and protozoa as major causes of loss of aquarium fish. Aquari-Cycline is active against wide range of rampositive and Gram-negative bacteria including Aeromomas and Pseudomonas sp. At the recommended dose rate Aquari-Cycline is bacteriostatic against Bacterial Septicaemia, Coldwater Disease, Columnaris Disease (mouth fungus, cottonwoll disease), Furunculosis, Fin & Tail Rot. Treatment at the recommended dose rate for three days with subsequent partial water changes has not effected nitrification. However prolonged treatment of mature aquariums with antibiotics will affect nitrifying bacteria, the use of a separate treatment tank is recommended where only a few fish are infected.