I can't really help with the fan issue because I've never used them before but I'm sure someone will pipe in. As far as the silicone...just make sure it is 100% silicone. If you're using it for your background get the black or brown. If you are using expanding foam for a background just remove your Exo Terra background and start on the glass. I spread a thin layer of silicone on the glass before I did the foam just to black it out. Once the foam has completely cured then cover with silicone and mash on your choice of substrate. Just remember with silicone the substrate has to be dry to stick. Then vacuum off the excess and your done. Check out Johns article on how to build a Viv and he talks about this. With this you can carve out places for your plants on your background.