would a 50 gallon water change a day help?
Not likely.
Your best bet is to turn off the filter and fill a syringe with hydrogen peroxide and blast away at it.
Many people feel Hydrogen peroxide can kill fish, and there was one incident when a fish died after a h2o2 dosing, but I'm not convinced it was related.
I've poured probably a half cup of it or more into a 20 gallon tank with tiger barbs and no ill effects were seen.
It breaks down into oxygen pretty quickly once you turn the filters on, and if the filters are off then it wont really be in contact with any animals.
Anyway, turn the filter off and attack the BGA with h2o2 several times a day for a few days, it should start to die back.
You can follow it off with a maracyn regimen afterwards if you want to be sure its gone.
But its possible it was introduced via tap water so I don't know that it would matter, it might just come back.