Itch should not be confused with flukes or Ichthyophthirius, even though the fish does dart about obviously suffering from some irritation.
The actual result is a microscopic organism in the water set up via an excess of uneaten food, and excretions being allowed to remain in the tank.
To cure, add 1/8 grain of permanganate of potash per gallon of water to the aquarium; it’ll not be essential to remove any fish or plants.
Leave for two or 3 hours, then remove any settlings in the bottom of the tank with a siphon, emptying the aquarium by a half.
Make up the deficiency with clear seasoned water. The slight shade of pink discoloration remaining will eventually clear.