I am thinking of adding some rocks to my 29 gallon tank and they are pretty big. It is a 29 gal glass tank and i am concerned about the weight any thoughts on what i could do.....
It should be fine. Tanks can handle huge amounts of weight as long it is not all to one point. If you have gravel, that may help spread out the weight more evenly. If you have to, "lighting grid" can be used. Its also called egg grid and can be found at your local Home Depot.
Yup, you may want to wash it a bit just in case there is a bit of dust on it. It is used in sumps, and holds up live rock (saltwater), so it is aquarium safe.
Egg crate/grid is great for spreading out the load (what you're trying to avoid is a tiny point load on the glass). I've heard of folks using thin pieces of expanded polystyrene as well - it floats, but under a very large rock that wouldn't be a problem.
Take care that if the rock rests on the side/back glass there isn't a point pressing there either.
I use egg-crate to hold down the lower layer of my substrate, which has mineral rich stuff, unlike the top which is just epoxy coated sand. Works ok for now, but it will be all mixed up in a couple years. The eggcrate doesn't seem to harm my fish, but then, I've had all sorts of plastic crap in my tanks...action figures, bottles...