I just used a razor blade and scraped mine off, just as if it were algae build up or something. It's easier when it's wet to get the hard scale off this way. You can do it when your tank is full and do a water change right afterwards. A schotch brite pad or dobe pad works ok, but I like the razor. It's faster with less muscle involved and I don't end up shloshing the tank water out.
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