"Is the blasting grit sand rough and scratch tanks? - This was controversial prior to me getting this. However, after going to the store seeing it and having it in my tank, it's just like anything else. Its not going to cut your hand if you touch it. It will NOT scratch your tank as long as used with proper care. Just as you would with gravel. You wouldn't put gravel under a magnet cleaner would you?"
Quoted from an eight page discussion
about it, which was the first result when Googling "black diamond scratch aquarium". Also has an example price, $8.99 for a 50lb. bag. I'd definitely gather as much info and opinions about it as possible prior to making a choice.