Hey guys, Between our 2 fish tanks, there are some difficult stains to remove. They feel hard, and I know on my established tank it occurred when fish splashed water which ran down the tank, resulting in the mark that I can't remove with anything. If I was to assume what they were, I would say it's probably calcium. I wasn't sure what to get, I was thinking about CLR but I wanted to save the $5 on it if I didn't need it. Thoughts?