Help my oranda goldfish started showing black colouration under its head. I'm not sure if this is normal or should I start treating it for black smudges.
20 gallons tank, 2 oranda goldfish, 1 platy and 2 nerite snails, tanks is cycled, 40% water change every week, saki hikari is their main diet and some spinach, I dont know the ammonia levels since I dont have a test kit. Fish is still active and eating.