Find a local hobbyist with the know-how, that will be the best way - diy strictly from two-dimensional internet instructions will not give you a show quality tank.
you probably only need 3/8" not 1/2", there will be a very slight bow with 3/8" however. front, side, bottom and back panels can all be 3/8" the top need only be 1/4" - arguably even the bottom can be done in 1/4" without problems as long as it is supported well (in other words, full length plywood top).
Black acrylic in 1/2" is verrry expensive.
Rounded from corners can be acheived fairly simply (my friend does it anyway with little problem) and will save you 2 extra seams to try and get perfect. I don't know what he use for a heating element, I am sure you can find that out. He builds a plywood box (4 sides, the fornt of the tank, bottom and sides) then heats and bends the acrylic at the desired points and uses the boxes 90 degree angles to get a perfect angle from front wall to sides.
just some ideas to get you going.
