From this thread https://www.fishforums.net/threads/what-happen-to-my-goldfish-back.456750/ The “disease” was treated. But it happened again (lower picture is before then it was treated and happened again) ammonia 0 nitrite 0 nitrate 10 do 75% daily water change for a week when I plug the filter the next day it happened again. @Colin_T @Deanasue @Byron