A month ago I decided to build a shelf for our turtle to sleep on in my tank similar to what greygoose had done with his tanks. http://www.redearslider.com/forum/viewtopic.php?t=6372
I cut the the shelf part and the rail pieces for it to sit on but on mine I put railings on top and bottom for the shelf to slide into to hold it into place. I cut the shelf with at least 2-3 millimeters room on either side. A month later when I went to clean the tank and slide the shelf out I couldn't. The shelf had expanded so much that it was actually bowed in the middle. Obviously not good since it was putting pressure on the tank walls. I managed to get the shelf out and shaved another few millimeters off the end of the shelf so it would fit again.
I didn't realize that plexiglass expanded like that.

