Malachite green (for fish treatment), Sumy region.

Malachite green is a synthetic acid-base dye. It is a shiny dark green crystal soluble in water and ethyl alcohol.
Application as a medicinal preparation in reservoirs:
During the treatment, they take 10 mg. crystals of malachite green and 15 mg. 0.1% solution of copper sulfate. Both drugs are diluted in 100 liters of water. The "healing" water is brought into the aquarium every 7-10 days, the treatment continues for a month and more.
During this time, the water temperature is maintained at the level of +28°С. The pH values ​​should be within 5.5 - 6.8.(//tractor-service.com)
Application:
as a reagent in microbiology;
as a dye in the food industry;
in the treatment of aquarium fish from infectious diseases.
In aqueous solutions, in a strongly acidic environment, it acquires a yellow color; at pH = 0.1-2.0, the solution turns blue-green; in a strongly alkaline medium at pH > 11.6-13.6 - the solution becomes discolored due to the addition of hydroxyl with the formation of triphenylcarbinol.