The plexiglass mirrors come in sheets and are lighter than glass. Mine actual sit on the bottom frame and then two little pieces of tape on top have held them in place nicely. I have had not problems with fish being upset with them - I have bettas in both tanks and I have yet to see a betta flare. Occasionally a molly will seem curious for a day or two, but they soon ignore it entirely. It does make it a little difficult to count fish - like neons.