My other goldfish died I don't want to loose this one
What are the tank stats? Size, chemical readings, etc. Also when did this start happening? Any details help.
Today 55 gallon never done a chemical test
If you could test the water for the water parameters it would be really helpful. Also is there any other fish in the tank?
Cory and a pleco
Goldfish are cold water fish whereas the cory and pleco are tropical warm water fish. What temperature is the tank? edit: I just want to add that cories also need to be in groups. In a 55 gallon I would get 6-10 of the same cory species. If the goldfish is a common goldfish, it belongs in a pond unfortunately. They get a foot long and are really no different than Koi in my opinion. And the pleco...if it is a common pleco, will outgrow the 55 as well..... You aren't asking about stocking, however, the health of your fish are very much affected by the water quality and the temp of the water...
pleco and goldfish shouldnt be together, the pleco will eventually notice the goldies slime coat and try to suck it off the fish