Yuppers it's safe... "plexiglass" is just a brand name for acrylic... Just make sure to cut them a 1/8" shy of the width you need and use a good silicone such as momentive rtv103 for placing them in a glass sump... Acrylic absorbs 1 to 1.3 percent "depending on brand" water over given time and swells which can pop seams on a glass tank when wedged in tight from the beginning....