Disclaimer: I am not a professional acrylic craftsman and am not responsable for 60 gal of water, rock, sand, snails, fish, coral, ect, ect, ect on your floor. (or your downstairs neighbors' cieling.
1/4 should be okay if you reinforce your seams with square stock and get REALLY gap-less joints.
Make sure you cut a hole in the top, not just piece 4 pieces of acrylic together to make the frame.
Go for it, and make it. Just sit it in your bathtub for a night to make sure...
Just my lowly opinion.
B