I'm sure you really don't want to (I wouldn't want to either) but if this was me and simply adding rocks to the tank causes the stand to start splitting I would IMMEDIATELY break down this tank and build a good stand that can sufficiently support the tank.
Even if you don't have the funds or means to build the stand right now I would rather have it setup on the floor than on a stand that is rocks distance from it's weight limit. If that tank implodes your stand (and that's a very good possibility if simply rocks will splinter it) you are going to have more than just a d.i.y. stand bangin' your pocket. You'll have a new 150g tank, and livestock bangin' your pocket.
