Unfortunately the builder is unknown (I purchased the tank used) as is the type of acrylic. The cracks emanate from the corners of the brace cut-outs (my tank top picture is posted here somewhere, but it looks like an old 4 pane window with basically a solid top panel with 4 16" x 30" cutouts in it) - the corner radius on the cutouts is less than 1/2", so that seems to be where the stress builds. The material is I believe 3/4" but may be 7/8", and that is same on top, bottom and all 4 sides.
I'll see if I can get a pic of the newer crack just to show what I mean.
Mark