The look of seamless round tanks that can be viewed from all sides can be amazing in many rooms. While round fish tanks seem more difficult to build, a few shortcuts will give you a virtually seamless tank. The hardest part is selecting the right materials.
Buy an acrylic tube in the desired thickness, height and diameter. Standard diameters go up to about 20 inches (500 mm). Larger tubes cannot usually be found in hardware stores, so for those sizes you will need to order the tube from a plastics company.
Ask the company you buy the tube from to sell you a matching bottom piece. The round bottom panel should fit exactly into the tube. Get the largest thickness available, so that it supports the weight of water. Some stores will glue the bottom piece on for you for free or for a nominal charge if you ask. Be sure to specify that you will be using the tube for a fish tank, so the sealant must be waterproof.
Place the round bottom panel into the tube and line them up perfectly. You can add sealant by placing the tank on a flat surface and first using a waterproof sealant, like WeldOn, to fill the inside seam. Make sure that it does not seep through the other side onto the surface. When the inside seam is dry (about 3 hours), flip the whole tank over and apply sealant to the outside seam.
Prevent the sealant from seeping through the seam by taping the bottom panel to the tube and laying the tube on its side. Start filling the inside seam slowly using a syringe, slowly rolling the tube to get all sides. It takes about 30 seconds for the sealant to set lightly, so work slowly enough that you do not get drips as you roll the tube. For extra protection, fill the outer seams as well.
Let dry for at least a week before you add water. You should do a leak test before adding fish. If there is a leak, simply repeat the seaming process in the area of the leak.
Buy Plexiglas in the desired height and with enough length to wrap around the base you have set. Thicker Plexiglas is hard to bend, so choose a thinner piece, but make sure it will support the pressure of the water. On round tanks, 1/4-inch Plexiglas is standard on tanks up to 20 inches high, while 1/2-inch Plexiglas should be used for tanks up to 30 inches tall. For taller tanks, speak with the plastics store to make sure you are using the correct grade and thickness of acrylic.
Use a strip heater to heat the Plexiglas until it bends. Since you are making a round tank, you should lightly apply pressure on each side until it reaches the correct angle. Try bending it around a round object to make things go faster.
Clamp the Plexiglas on both sides at the seam. Make sure ends touch perfectly. An uneven seam may create bubbles in the sealant.
Roll the tube until the seam is on the top (parallel to the floor). This will keep sealant from dripping.
Apply sealant and let dry. When it is dry, place the Plexiglas tube on a round piece of acrylic and seal the seams.
Use WeldOn or another acrylic solvent initially to seal the seams. A good application of this should make the aquariums watertight, but you can add extra protection, especially if the seams were glued by someone else.
Apply a layer of acrylic cement to the seams. Do not get the cement anywhere else on the tank or it will scratch or cause other damage. The extra layer of cement will fill any holes from uneven seams.
Let dry and reapply acrylic cement to the inside seams. This extra layer ensures that the fish tank is watertight. Keep the thicker application of cement from inching up the sides of the tank, because it will be seen. The bottom will be covered by substrate.
Most plastic shops will bend the Plexiglas for you and seal or clamp it in a round shape. If you are planning on building tanks regularly, you should form a relationship with a shop that will shape the acrylic for you. Tall tanks with thinner acrylic will bow slightly when filled with water, which is normal, but a thicker plastic can be used to minimize bowing.