I don't know what kind of climate you live in, but if it's warm enough, you may consider setting up a temporary pond for your goldfish. They're really too big to be in a 55 gallon tank (as you probably know, but can't upgrade...I have considered a 75 and 90 numerous times, but a 55 is my largest tank too right now) and 50% water changes weekly, as you should be doing, are tough to do without a Python or other similar tool.
I had a stubborn cause of fungus on my black moor. I set up a little outdoor pond for him from a child's 5 foot wading pool. I planted it with floating hornwort, and floating Salvinia (I also added some houseplants that I knew could take wet feet). It's taken off with very little help from me. He looks fantastic now. Fresh air, sunshine and loads more water than he had in his 30 gallon tank was all it took to cure him

The water stays very clean (undetectable nitrates) from all of the plants.
Here's a picture:
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I'll eventually have to bring him in for the winter, but I know he's loving his summer outside!