So the 10 gallon nursery tank does not have a heater or filtration? I don't have a light on my quarantine, either.
What's the current food and how long have you been feeding that?
Go ahead and check the water parameters. We can't help much without knowing ammonia, nitrite, and nitrate levels. Having pH and hardness would be helpful, too.
The fungus may have progressed to a bacterial infection. Consider tetracycline or erythromycin.
One of the best treatments for any ailment is clean water. Especially if this tank doesn't have filtration, you need to be doing water changes at least every other day.