Plexiglass, Acrylite ... it's all acrylic, just a brand name. The thing to watch for is cell-cast verses extruded. I think HD & Lowes, is extruded ... not 100% sure, but it looks like it. I'd have to cut a piece to be 100% sure. Cell-cast is much stronger and is not really needed unless it's going to hold water (needs some strength).
Either type will still curl up on the corners if you cover an aquarium (tank/sump) and heat the water to support warm water life. If this happens, just flip the piece over every few weeks. It's the combination of the moisture and the heat (from the water) that causes this. I have even tested this on 3/4" peices and it still alters the flatness.