If I may interject here...
Even though the pictures are not clear, what I see appears to be a possible tumorous growth beginning to spread over the eye. The fact that it is raised and "barnacle like" as you put it, tells me that it is indeed excess tissue, but it doesn't appear to be hanging.
Now, the bad news....
It is impossible via internet to know if this is a malignant tumor, a progressing infection, or even a benign tumor, which is possible. Without a biopsy of the infected tissue there is no other way to know that for sure.
After reading your description of how these fish are being kept, my first guess would have to be that the issue is in large part due to water quality. To know that a 1 yr old fancy goldfish still fits into a 10 gallon tank is heart breaking, but to hear that 2 of them AND a standard pleco still fit into a 10 gallon after a year, that tells me this tank has had serious problems for a very long time.
Heavy nitrate levels are going to be unavoidable in this kind of situation. I am very curious to know how often and how much these fish are being fed, along with what kinds of foods and how often water changes are being done, and how much water each time. Heavy nitrate levels over a period of time have been known to cause these kinds of problems, and while I can offer to help you clean it up and give this fish the best chance to heal... I can't offer any help with treatment of the problem other than in fixing the conditions that caused it.
If its of any comfort to you, I took in a ryukin a number of years ago, full of tumors all over the body, covering both eyes. Everyone around me thought the fish was a goner. This fish was about 5 inches long and 2 yrs old when I took it, and coming from a 30 gallon tank with other large fish. After about 6 months in a properly set up and clean 75 gallon tank the tumors began to disappear. Slowly at first, but then faster and faster. That fish lived another 3 yrs after I took it, and it had quality of life left before it died of internal injuries, all of which appeared to be caused by being kept in a dirty tank that was too small. This fish should have lived about 20 yrs.
I urge you to find a way to either get these fish into a MUCH larger tank asap, or to find them new homes in proper size tanks. If kept under these conditions there is no way to avoid an agonizing and early death in them all.