Well i have 22 fishes and well rite now i just noticed that 3 fishes have some n...
The general rule is one inch of adult fish size per gallon of water.
It sounds like your fish have Protozoa (Skin Parasites). Quarantine any fish that show signs of the disease. Treat water with Formalin, Malachite Green, Acriflavine or copper sulfate.
Test water quality frequently; conduct periodic water changes; ensure good aeration and filtration of the aquarium water.