According to Charlie Ingham, director of Complete Koi & Aquatics, the small white specks that appear on the fish’s body and fins are due to a condition known as Ichthyophthirius multifiliis or more commonly referred to as ich.
It is a fungal infection that spreads quickly throughout an aquarium and without effective treatment, it is likely for the fish to die.
Solution:
Fish with ich should be treated with specially formulated medicine (can be bought at pet stores) to kill the virus.
Follow the directions of the medication carefully.
For example, 10ml of Azoo’s Anti-White Spot should be added per 100ml of water every 24 hours. This should be done until all white spots disappear.