Yup! Or you can use colored if you'd like.

It won't hurt your fish as long as it's completely dry when you put it back into the tank. Also, it will flake off after a while, and usually it's caught by the filter, but I suppose there's that possibility. Not that it's ever happened to me though.
I put a small 1"x1" piece in. It takes a reeeeaaallly long time to dissolve, so I also scrape some off in the tank with my fingernail to make a sort of powder.
Some people have had problems with it rotting and throwing off the water stuff, but I've had luck with it so far. And it's really cheap.

You might also try Kent's Liquid Calcium. I saw it at Petco yesterday for $8.