Strait cuts in acrylic are easily made by scoring repeatedly with a really sharp utility knife and then snapping the two pieces apart. To do curved inside cuts, I used my hand-held jigsaw and a fine tooth blade intended for cutting plastic pipe. You have to go slow, or the heat causes the stuff to melt. Wetting the area around the blade helps, but you have to be very careful not to zap yourself.
I'm not sure that 1/4 inch will not bow from the heat of the tank and the lights. I found 1/8 inch acrylic to be completely useless as a cover for a 10 gal
. Hope you have better luck with the thicker material. Otherwise, you might have to go to an auto glass shop to have something made for you.