i forget who originally posted this in this area 15 or so years ago... i've split it into two parts... one with the instructions and the second with the materials and layout plans...

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How to build a 100 and a 240 Gallon Plywood Aquarium Tank

Your first step is to assemble all of the materials listed on the materials sheet accompanying this article. When buying plywood, it is recommended that you purchase it from a lumberyard that also has a good mill shop. Unless you are unusually talented in woodworking or cabinet making, it is not advised that you cut your own lumber since the bottom and end sides must be cut very accurately. Any irregularities from end cutting will present serious problems when the aquarium is assembled.

The tank must first be assembled using the finishing nails to hold it together while drilling the screw holes. The 100-gallon tank can be put together by one person, but the 240-gallon will require two people due to the bulk and weight.

Start the construction by selecting the bottom of the tank. For the 100-gallon tank, this is the 21" x 46-1/2" piece, or for the 240-gallon, this is the 24" x 94-1/2" sheet. Stand the bottom on edge and make certain that the "A", or good side, is placed on the inside of the tank. Take one of the pieces for the side, place it over the edge of the bottom, and secure it with two or three finishing nails. In order to allow for easy removal, do not drive the finishing nails in all the way. Make absolutely certan that the pieces are in perfect alignment. When this has been completed, repeat the process for the other end piece.

Next, take one of the 24" x 48" pieces and place it over the partially assembled bottom end. Line it up and secure it in place with the finishing nails. Do not drive the nails in all the way. Repeat the process on the other side. You now have two strips of plywood left. These pieces are 48" long and will have to be cut so that they are also 46-1/2". The accuracy of this cut is not important and can be done at home. Square it up and fit it inside the top lip of the tank. Repeat this process on the other side.

Now draw a straight-line 3/8" from all edges on both sides and the bottom side of the end. Then, starting from about 2" from the end of all the lines, make a pencil mark every 3". This mark will be where the screw holes are drilled. At most lumber companies and discount stores you can buy a drill with a countersink attached. Ask the clerk to provide you with the proper size for a #10 screw. If you cannot find a drill with an attached countersink, it will be necessary to buy a separate countersink. This should not be necessary, however, since the combination unit is readily available. Once obtained, drill all the holes that you have indicated with a pencil mark. Do not drill too deeply. The head of the screw, when set, should be approximately 1/16" below the surface of the wood.

Mark the sides so that the tank can be put back exactly as it was, then disassemble the unit. Shake out the sawdust and sand off any irregularities caused by the drilling. Do not over sand and change the contour of the wood.

Mark the sides so that the tank can be put back exactly as you assembled it before, paying attention to your assembly indicator marks. This time, however, a bead of Elmer's waterproof glue is applied. The finishing nails can now be driven in all the way and the tank screwed together. It is a very tiring, slow, and tedious job if you are using a standard screwdriver and not an electric drill with a screwdriver attachment. The screws are staggered between the 2-1/2" and 3" length screws. Start the row with a 2-1/2" screw and then alternate. Be sure the tank is perfectly aligned before completing the final assembly. Wipe off the excess glue before it hardens.

Once the glue has hardened, the next step is to cut out the front for the insertion of the glass. It is only necessary that a 2-1/2" lip be left on the front side, so draw lines in 2-1/2" from all sides. If you desire rounded corners, use a soup can in each corner and draw the necessary curve. Drill a hole on the inside of the line of sufficient width to allow the insertion of a saber saw. Go slowly and cut out the front of the tank. Don't force the saber saw by going too fast--taking your time will result in a nice, even cut. Have someone hold up the plywood as you complete the cut to avoid the weight of the inside panel tearing the lip. If you didn't make the cut as even as you would like it, now is the time to sand it down and improve its appearance.

On the cut ends of the plywood, there will be gaps of holes inherent in the manufacturing of this product. Fill in these holes, whenever noticed, using plastic wood. When dry, sand the filled edges to a smooth finish. Shake all the sawdust out of the tank and dust it carefully. A vacuum cleaner hose with a brush attachment makes the job easier. You can finish the fine dusting with a tack cloth just prior to painting the inside.

The best paint found is Pratt & Lambert, which is readily available. Be sure to get Gloss Activator since it produces a much better finish for this use. Do not use any paint that does not require the mixing with an activator. There are paints that don't require mixing which have the word "epoxy" on their label, but they are totally useless for this type of work. You must use an epoxy paint that utilizes an activator or catalyst.

When mixing, follow the directions and mix only enough for one coat. Once the paint is mixed, it cannot be stored and must be used within a few hours. It is only necessary to paint the inside and the edges with epoxy unless you want to--it is only a waste of money. After painting the first coat, allow the tank to dry for 24 hours, then re-coat. Some aquarists use only two coats. However, three coats are preferred. The latter would be necessary if you were going to use the tank as a marine aquarium. The 1/2-gallon of paint is enough for three coats.

After the final coat of paint is dry, many aquarists put a light beading of silicon aquarium sealant in all corners. In most cases, this is not necessary but applying it will serve as insurance against leakage, should there be a bubble or crack in one of the corners.

The final step is the insertion of the glass. Cut the end of the silicone sealant spout to its widest opening. Place the tank with the cut side down and run a continuous bead of silicone aquarium sealant all around the lip. Center the silicone beads so that they are about 1/2 way between the edge and the sides of the tank. Take the glass, tilt it and set it on top of the silicone bead. Press it firmly down on all edges. It is a good idea to take a couple of pails of water, or other weights, and set on top of the glass to keep pressure on the silicone beads until they are set. Let it cure for a few days, then remove the weights. Trim off any excess silicone sealant and test the tank. This is accomplished by filling with water. If no leaks appear then drain, the water and you can put the tank in service. Should any leaks occur--an unlikely occurrence--a little silicone aquarium sealant will quickly seal it.

If the aquarium is to be used in the basement fish room, a coat or two of latex paint will be satisfactory for the exterior. If, however, it is to be used in a more formal setting, the tank can be faced with stained or varnished paneling or other desired finish.

This is an economical way to acquire a large tank. Given proper care, the life of your new tank should be a lengthy one. I have 100-gallon tanks that were built 20 years ago and are still in service!

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"The soul of a small kitten in the body of a mighty dragon. Look on my majesty, ye mighty, and despair! Or bring me catnip. Your choice. Oooh, a shiny thing!"
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