What troylee said. Silicone will harden to them. You just can't make a tank that way. So , for example, if you needed to make an acrylic wall, you can use silicone on the back part of your tank. You just cant make it structural. I guess I should ask what your wanting to do?
I cant find the rtv. My local hardware stores dont seem to have it. If you dont need a lot, Lowes and Home depot sell an aquarium silicone. It comes in a tube and cost a little more. If you need a lot, the rtv is what comes most recommended. I just cant find. If you can't find either of those, you have to find a silicone that is not used in bath or shower. They have a fungicide or mildew killer type stuff in them.
Use Loctite Stick-n-Seal. Its tensile strength is three times stronger than silicone when bonding to acrylic. It is also clear and water proof. Next time you are at Saxon & I-4, stop in at HD and get ya some.