A blade designed to cut laminate should do it. Be sure to invest in a good quality carbide tipped blade, as the cheapo loco steel blades will heat and warp and totally ruin your day.
What I'd do here, given what's at stake for this cut is to do some serious prep work... the cut will take seconds, but spend a couple hours preparing. The big risk is that the saw cuts ever so slightly downhill or uphill, so what you'll get when you get around to meet where you started is a misalignment that will look crappy and prevent any bracing.
What I'd do is get (2) 10 foot and (1) 8 foot (cut this one in half) select 2x4s. Be sure to invest in the highest quality, straightest possible boards you can get. Obviously don't buy oak or something, but good quality pine boards checked for warpage and true is good. Even better is if you have access to a table saw where you can rip them to "true them up" to dead-on-flat.
Cut them to form a box around the perimeter of the tank, minus about 1/16 on each side. Then use deck screws to assemble the box around the tank. With the boards being 1/16" shorter, as you tighten the screws (CAREFULLY!) it will "clamp" onto the tank. Line it up before tightening so that when you rest the saw against the boards, the blade falls right where you want it to.
You should see where I'm going here... the boards will form a circular "fence" that you can ride the saw along as you cut. Doing so will ensure that the blade ends in the same spot it began, and ensure a nice, square, professional cut. You probably ought to have a helper watch while you cut to be sure the fence doesn't "slip".
One other thing... be sure to attach something to the bottom plate of the saw. Part of the base plate will ride on acrylic that will be part of your show tank, you don't want that metal to scratch. A piece of cloth, held in place with double sided tape should do it.... I'm sure you can brainstorm something.
The best thing to do to check if you need bracing is to just fill the cut tank up and watch for deflection. If it's excessive, cut a few 3-4" strips from your cutoffs to glue to the top.
One last thing... you may want to make a few test cuts along the top of the tank (the areas you know will be scrap) to get a feel for the proper speed to cut. Going too slow will cause the acrylic to melt... too fast can cause chipping.
Just some thoughts I'd toss you.
Is this the same dude in your PMs? I hope he's paying you!
