The most recent advancement in control of external organisms causing fish disease is the pond-treatment method. It is an improvement over the hand-dipping method because fish are not killed by handling, fewer men are needed to carry it out, and the pond itself is treated along with the fish. All external parasites are controlled by a treatment with formalin (diluted to 1:4000) for an exposure of 1 hour, except Ichthyophthirius multifiliis, parasitic copepods, and species of Saprolegnia. I. multifiliis can be controlled by holding affected fish in swiftly running water for several weeks or by chemical treatment of the water for the duration of the life cycle of the parasite. Free-swimming larvae of the copepod Salmincola edwardsii can be killed by chemical added to the water; because of the parasite's long life cycle, treatment would hardly be feasible at large hatcheries. Saprolegnia sp. (fungus) can be killed readily by dipping fish in a 1:15,000 solution of malachite green for 10 to 30 seconds, or by introduction of the chemical into the pond for 45 minutes at a dilution of 1:180,000. Fish eggs can be kept fungus-free by semi-weekly treatments with a 1:200,000 concentration of malachite green.