I have a pair of 5inch retics this morning I noticed one had a patch of black spots on the upper side of its left wing. My water is 0ppm ammonia, 0ppm nitrite and 20ppm nitrate. I test my water twice a day and it is a mature tank and filters. They are both very active and eating like pigs. I have tried to include a photo but the spots are impossible to see due to the fishes pattern. I had to remove a snail from the fish two nights ago. Could that be enough to remove the mucus and form an infection????? Any help would be fantastic as I dont want either to suffer.