Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If rocks had come from a wet location (pond, stream...) then bleach would kill any disease or parasites. Not needed for roadside rocks. Boiling is the same. Good to kill stuff, but I don't know of any fish parasites that thrive on dry rocks.
Roadside = petroleum runoff from the vehicles. I would use a cleanser that is made for washing your hands after working on your car. Scrub a LOT. Rinse well.
Put the rock in a bucket. Let it soak for a few days.
If it is rust, it will look a bit different after being under water. The rusting will be going on, and you may be able to scrub off some, or at least it might smear around when you scrub it.
If it is really rust, but just a little bit, then ignore it, unless your tap water already adds way too much iron to the tank. I have a few spots of rust on several rocks and there are no problems. About 2-3 square inches in a 125 gallon tank. Maybe half a dozen dime sized spots, and a few narrow skeins. I have Bichers, Clown Loaches and a Black Ghost Knife (and other fish) in this tank.
If it is lava, then it will not change when it sits in water, you will not be able to scrub it off or change it, and is usually no problem in a tank.