Just a guess am I scraping coraline with my metal scraper when it is on an undesirable surface such as the sides of my tank? Or is there a better way, that won't scratch the glass?
If it’s glass- metal is ok if your careful. I use a old card like a credit card or a old hotel room key card. That seems to be soft enough not to scratch and hard enough to get coralline off.
I also use it on acrylic but with more caution as the coraline between the card can still scratch.