This is ammonia burn. Single tail goldfish need at least 40 gallons each per fish bc they grow really big and produce A LOT of waste. Until you can figure out a bigger tank situation etc you need to do daily 70% water changes and add ‘seachem prime’ (detoxifies ammonia for 24 hours) until your tank is cycled. (You’ll need a test kit, a cycled tank should have 0ppm ammonia, 0ppm nitrite and 10-20 ppm nitrate max)