Either find a plan for a fish tank or design one. Fish tanks come in several shapes and sizes, but square and rectangular are the easiest to build. Square fish tanks require five equally sized pieces of Plexiglas. Rectangular fish tanks use three same-sized rectangular pieces and two square pieces the same height as one of the rectangular ends.
Calculate the full size of the Plexiglas required for the project, and then purchase either a large sheet, or precut sections. Most hardware stores cut Plexiglas for an extra fee, and it is worth the money for anyone who does not have a heavy-duty router. Purchase aquarium-safe silicone, solvent cement such as Weld On #4, a solvent bottle, duct tape, screws, clamps and a 2"x4" board.
Use a router saw to cut the Plexiglas to size. Measure the thickness of the Plexiglas, and then calculate that into the final width of the fish tank. Cut the 2 x 4-inch board into the length and width of the fish tank, and screw it together to create a frame.
Double check the edges of the Plexiglas for burrs and nicks, and then place the bottom plate into the frame. Enlist the help of a friend if possible, and place a connecting glass in place on the fish tank. Clamp it in place. Repeat the process until all five Plexiglas pieces are in place. Duct tape around the Plexiglas to hold it in place. Use the solvent bottle to apply the solvent cement horizontally along the joints. Wait for four hours until the cement dries. Then, carefully flip the tank on one side, and add solvent to the horizontal corners. Use a block to support the top glass if necessary. Repeat the process on the other side. Wait for four hours until the final seams cure. Apply silicone to the inside of the seams to finalize the seal.