Fish hatcheries and fish farming play an ever increasing role in supplying the world's demand for fish. To effectively supply this demand fish farms must practice intensive farming techniques where more and more fish are farmed in the same or smaller tank volumes.

Of course as the fish density grows so does the risk of infection by water-borne bacteria and viruses.

Ozone is the ideal disinfectant for aquaculture because of its ability to kill bacteria and viruses without leaving any residue.

Ozone is an effectivefor aquaculture water treatment that:

Oxidize organic matter such as fish excrement bait etc effectively removes organics pesticides discoloration and nitrates.  
Precipitate dissolved matter improves the effectiveness of biological and particulate filtration.
Allows the micro-flocculation of organic matter
Destabilize colloidal particles which are unaffected by biofiltration.
Sterilize and disinfect the water.

Ozone advantages for fish farming:
Reduced Water Usage
Faster Growth Rates
Reduction of Waterborne Diseases
Higher Standard of Environmental Control
Supplements other Treatment Processes

Moreover any excess ozone decomposes to oxygen and thus poses no health risk for the fish or the people who subsequently consume them.

Ozone is unlike such agents as chlorine or any of its derivatives oxidation with ozone leaves no hard to handle or toxic residues requiring subsequent complex treatment. In practice ozone immediately starts to attack the oxidizable components it comes into contact with. This property makes it a very powerful disinfectant. Because the process only leaves "oxygenated" products and oxygen it is particularly well suited for applications such as hatchery water where the presence of undesirable elements after treatment could have grave consequences.

Additional info: ozone geneartor is also effective for poultry farming.

The Reaction of Ozone with Ammonia

Mixtures of O₃ and excess NH₃ react at ~30°C to produce O₂, H₂O, N₂O, N₂, and solid NH₄NO₃. The amounts of the gas-phase products, relative to the ozone consumed, are 1.05, 0.31, 0.032, and 0.031, respectively. Neither H₂ nor NH₄NO₂ was produced. For [NH₃]/[O₃]₀ ratios < 50, the disappearance rate of O₃ was first order in [O₃] and increased slowly with increasing [NH₃]/[O₃]₀ to an upper limiting value of 0.21 min^-1, where [O₃]₀ is the initial pressure of O₃. As the reaction proceeded and the [NH₃]/[O₃] ratio passed 120 (or if [NH₃]/[O₃]₀ > 120), the rate shifted to three-halves order in [O₃] and was proportional to [NH₃]^-1/2. The reaction is interpreted as a chain mechanism with the heterogeneous decay of O₃ as the initiating step. Nitrogenous products come from oxidation of HNO with O₃, followed by reaction with NH₃.