With those stainless bolts in the acrylic windows every 4 inches I would think you could probably use a slightly thinner window than you normally would for that depth, so 5/8 or 3/4 would probably work. Most tanks don't have any mechanical fasteners and the acrylic isn't going to crack. Heck with those bolts 1/4" acrylic might hold, but I wouldn't try it.