Flat worms are generally between 1/8 to 3/16 -in. long, and coloured a pale grey, comparable to the body of a garden snail. They could be seen crawling on the glass of the aquarium.
Flat worms aren't enemies in the true sense of the word, but they're undesirable. They feed on microscopic organisms, and if placed in clear water will soon perish via lack of food. They could be trapped on a piece of raw meat.
You suspend the meat by a piece of cotton in the aquarium. The meat should be guarded to stop the fish eating it. Should you begin the treatment in the evening you'll find next morning that there are fiat worms crawling over the meat, which you are able to then remove. Continue this treatment until the trouble is cleared up.