The recently developed ozone-like oxidant, Photo-zone, was studied for its effects on selected biochemical and biological parameters in a closed aquatic system. Parameters investigated included improvement of water quality, inactivation of bacterial pathogens and destruction of fish ciliate parasites. Experiments were conducted both in vitro and in vivo, using 10-gallon aquaria containing channel catfish fingerlings. The degree of efficacy of Photo-zone treatment differed considerably between the two test systems. In vitro results showed nitrites to be rapidly and completely eliminated the bacterial fish pathogen Aeromonas hydrophila to be rapidly inactivated and the ciliate Tetrahymena pyriformis to be completely destroyed. In vivo results showed variable elimination of nitrites, a 99% reduction of circulating total bacteria and variable prevention of Ichthyophthirius infection. The authors conclude that Photo-zone might be a viable alternative to classical ozone treatment in certain specific aquatic applications; however, additional research is necessary to further define this and other systems.