Vinegar! I used 5 gallon no water first to let the bottom section get the full dose. I then filled it up with water a few hrs later and ran it through the system till the day. Most came off & the other hlaf was easy to remove cause it was soft after the next day.
Last week I got this 125g
Right now as Im taking out the water cause I need to do some work- paint the back glass..etc,...