I disagree about the plexiglass. You'll still have sharp edges once you cut it and it makes a mess. (The only way I've successfuly cut it was with a circ or table saw). It will also scratch and haze over time. If you take one tank into a glass shop and ask them to make 2 pieces that will fit, the rest is easy. (I say bring in the tank so they understand the exact measurement and how it needs to fit). They'll also knock off the sharp edge but you'll still have a flat edge to mount to the tank.
Get 100% silicone, about $3.50 a tube, you'll only need one. Use a caulking gun to caulk a bead on each side of the glass all the way around where it meets the tank. Use your finger to lightly press and smooth the bead of caulk. Wipe excess on something you'll throw away cuz this stuff is sticky. Let dry and your done!
No disrespect towards previous poster, I just don't like cutting plexi!
DZ