careful with that bleach ratio, i did a 1:25 bleach on my jet black rocks and ended up with a bucket of offish grey...looks like slate now though. Even some of the "natural" rocks are either colorized that way or will bleach out some. I second the oxy clean idea.
chlorine and fish do not mix!!!! If you go with bleach follow up with soaking in freshwater treated with a dechlorinator over night. I like the idea of boiling the gravel. Just my .02