does anyone know if plexiglass is a bad idea for inside of a fish tank... not sure what is cheaper, glass or plexi, but plexi is easier to cut to fit than glass... let me know what you guys think.
Plexiglass works just fine. However, if the divider (a guess at your planned purpose) is needed to hold the water back (with air on the other side) or if used to support something, it is best to use glass as the silicone won't really stick well to the plexiglass. If you don't mind a tiny bit of leakage from one side to the other and have eqaul pressures, siliconing a plexiglass sheet inside of a glass tank works great.
i guess i should have mentioned what i was thinking of using it for... sorry
my plan is to make a real rock background, but i don't want to silicone anything to the actual tank... so my thought was to get a peice of plexi and adhear my rocks to that... maybe scuff of the plexi so it is rouch and the silicone will adhere better and use something else to fill in gaps between the bigger rocks... then silicone a hook system to the top of background and have the hooks to keep from falling into the tank. and maybe a couple of rocks in the tank put against the background at the bottom to hold the bottom in place...
so with that said, if plexi is safe to use, what WOULD be better for this application, glass or plexi.
For your application, plexi glass should work fine. It is completely safe for under water use (most of my DIY sumps are acrylic-Plexiglass is really a brand name). If you give it a really good scuffing the silicone should hold on the rocks, as long as they are not too heavy. Many people have used PVC, acrylic or ABS sheets to make their backgrounds removable.