Katya was continuously changing the water and cleaning the fish tank. As she read in the articles on fish handlings, she decided to change all water and to fully clean up the tank after half a year.
Last evening, we fully cleaned the tank and filters. Sands were all changed. New water, having kept outside for some while, was put in the tank.
While I was at school today, Katya put all fish in the tank. Successfully, all fish swim happily in the new environment.