Aquariums made from glass or acrylic can be very expensive; however, it is possible to build an aquarium from plywood to save money. Plywood saves the most money for aquariums that are between 75 to 500 gallons, and has the added benefit of making the aquarium lighter. Typically, people are only looking through one side of the aquarium, so only one side needs to be made of glass. The back, bottom and sides can all be constructed from sealed plywood.
Sketch out a scale drawing of the aquarium including every detail. The plywood will bend and break if not supported by boards running along the back and under the aquarium. Include these in your drawing.
Carefully cut your sheets of plywood. The edges should be perfectly straight and each side should be one sheet of wood to help prevent leaks.
Cut 2x4s and make a frame for the bottom and sides of the aquarium. The outside of the frame should be the exact same dimensions as the plywood walls. You will bolt the frames together to form the walls of the aquarium. The plywood is not strong enough by itself to hold the weight of the water.
Cut 2x4s to make a skeleton on the inside of the frames behind the plywood. Insert them so that the long end is perpendicular to the plywood as this will help prevent the boards and the plywood from bending under the weight of the water. Screw the 2x4s to the frame with heavy duty wood screws or drywall screws.
Bolt the walls to each other and to the base. Be sure to use bolts for this as they will take the full pressure of the water.
Build the front panel of the aquarium by creating a thick frame of 2x4s bolted together. You can leave the centre open to create a place for the glass, or seal it with plywood and a skeleton just like the other walls for tanks you will look down into. If you plan to insert glass, make sure you make a lip on the outside of the frame where the glass will fit inside.
Coat the inside of the aquarium with four coats of epoxy paint. Sand with medium grit sandpaper between the first two coats, then use fine grit before the last coat. It is highly recommended that you also coat both the inside and outside of the aquarium to help prevent the wood from rotting. Let each coat of epoxy paint dry for 24 hours.
Add a watertight bead of aquarium-safe silicone to all seams inside the aquarium.
Add a support beam across the top of the aquarium connecting the front panel to the back panel. This will help prevent the wood from bending when filled with water.
In a safe place, fill the aquarium with water and check carefully for leaks. Do this outside so that if something breaks, the water will run off safely.
Build a scale model of the aquarium first to test your design.
Epoxy paint vapours are very toxic. Only paint in a well-ventilated area, preferably outside. If you add rocks to the aquarium, make sure they don't scratch off the epoxy on the bottom or walls of the aquarium.