Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Soda, like Coke is very acidic.
Citric acid crystals (grocery store, sold as 'Sour Salt')
Here is what worked best when I tried several methods:
WEAR GLOVES! like the heavy duty dish washing gloves.
Scrape off whatever is loose with a razor or other tools.
Wet the remaining white stuff with lemon juice.
Sprinkle on citric acid crystals.
Cover the area with a paper towel soaked in lemon juice.
Let it soak about 15-30 minutes, wetting the paper towel as needed.
Scrape as much off as will come off with a razor.
Other materials: Use with care, read the instructions about safety equipment like goggles and gloves.
CLR = Calcium, Lime & Rust remover, available in hardware stores.
Muriatic acid, a swimming pool product.
Rinse any of these well.