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Another DIY Plywood Tank Article
by Matt Coxe

When it comes to tank sizes, bigger is better! My fish were outgrowing my 33 gallon and I needed a bigger tank. The local fish stores wanted too much for their tanks. I had read about plywood tanks before and I do enjoy DIY projects. The solution was to build my own tank.

Before you attempt building a plywood tank, I would recommend reading not only this article, but everything you can find pertaining to plywood tanks. Typing "diy plywood aquarium" in a search engine should get you started. I researched for about 3 months before finally taking the plunge. I live in an apartment and failure was not an option, it had to hold water; it could not be a waste of time and money.

One of the reasons for this article is that I see a lot of people ask "what do I coat a plywood tank with?". I used and recommend a marine epoxy resin. The product I used is  from WEST SYSTEM  and is not a paint at all, but a 100% solids adhesive resin. I see some people trying different things that are probably a bit cheaper, and they may work fine, but I don't have the time and space to experiment. I know that this resin is made for the marine industry and has been successfully used for building water tanks. One thing with epoxy resin that you may not be sure of with other materials is that it is an outstanding adhesive and bonds to wood extremely well.

I did something a little different with my tank by using a layer of fiberglass cloth and laminated that with epoxy. At first I thought maybe a layer of fiberglass/polyester resin, until I discovered that epoxy resin can be used to laminate fiberglass, resulting in a better bond to the wood and a more durable finished product. Epoxy is also waterproof, and after researching on the Web, I have discovered that polyester resin is not. This step is not necessary and does make the job more complicated, but I have had experience with fiberglass repairs on race car bodies in the past, so I figured I would give it a shot.


I had the wood cut at the lumber store. It cost me an extra $2 and it was well worth it in time saved alone, not to mention that I don't own a table saw and using my Skil saw was out of the question. All of the boards fit together perfectly with no gaps in the joints.

I used 2" galvanized wood screws to hold everything together. I bought a box of 100 and used every single one. I spaced the screws 3" apart, and 2" apart on the front frame. In order to keep the screws straight, you will need a line to go by. I measured out 3/8" from the edge then used a straightedge to mark a nice straight line all the way around. Use care and drill nice and straight. You don't want the screw splitting the outer layer of wood and coming through. I put a bead of urethane construction adhesive on the joints before I screwed them together.

It's nice to have a helper in this stage to hold things together while you get the screws started. I used a flat board to to make sure I had everything lined up perfectly and put a screw in one side, then I put one in the opposite side. I used an alternating pattern as shown above. I started all screws by hand and used a drill to run them in. I then checked by hand for tightness.

Be sure to wipe off any excess glue from the joints. You don't want anything between the epoxy and the wood. I gave the wood a good sand with 80 grit paper to help rough it up and to remove any traces of the construction adhesive on the surface.

I used Oak and Poplar for the front frame. I thought that I would need some stronger wood to prevent the frame from bowing outward. After getting all the screws in I realized that this frame is much stronger than it looks. At this point the wooden box is strong enough to support an elephant. I jumped up and down on it in celebration! Try that with a glass tank.

This tank will have an integral hood. The water line will be about 5" below the top. I wanted the top open without a center brace, so after the epoxy was done I put some oak 1X2's along the top to prevent bowing. I was a little unsure about this working, but it ended up working great.


The Tools of the Trade:

•  Marine epoxy resin
•  A proper respirator
•  Lots of disposable brushes

The resin I used is 100% solids with no solvents. It doesn't smell that bad out of the can, but don't be fooled. I  highly recommend using a respirator and wearing gloves at all times while using this stuff. West System Epoxy is more expensive than some of the other stuff. I'll bet the cheaper stuff is equally as good for waterproofing a tank, but West System's pump  mixing system is very clean, convenient and accurate. Check with your epoxy's manufacturer for mixing instructions. I used a 2 cup method to be certain that the epoxy was thoroughly mixed. It is very important to mix thoroughly and accurately.

Get a bunch of brushes, because there is no saving a used brush. Throw it away. Don't even attempt to clean it. Use a fresh brush for each coat. Buy the cheap ones, but make sure they don't shed a lot of bristles, unless you don't mind bristles in your epoxy finish.


I painted the outside with 3 coats of marine enamel and cleaned the plywood with lacquer thinner to remove any wax and grease. Epoxy contains no solvents so oily residue really interferes with adhesion.

At right is the fiberglass cloth. Not all fiberglass is compatible with epoxy, so you may want to ask a retailer who specializes in marine products what is best for your application.

I trimmed the fiberglass cloth with a cloth cutting blade that looks like a small pizza cutter. These are available from most fabric stores. I stole mine from my wife. I used a straightedge for the cuts. This cloth is awkward because it's basically a weave of flat strips of glass fibers. You can cut it nice and square and it can turn into a parallelogram quite easily. I cut it the best I could and carefully laid it in the tank. I used a plastic squeegee (the kind used for autobody filler) to iron out the bumps and wrinkles.

Once I had it where I wanted it, I mixed some epoxy and started to wet out the cloth. For the first layer I basically just brushed it on very wet, trying to totally saturate the cloth. I then went over it with the squeegee to flatten it out. Properly wet out cloth is virtually transparent (see photo above at right). Any spots that aren't properly wet out are easily visible. All creases and air bubbles must be squeeged out. I did the bottom and back in one piece. The was a little tricky. Luckily epoxy resin has more working time than polyester, or I would have been in real trouble. This was a very cumbersome part of the project. Next time I will do this to each individual piece of plywood before assembly. This is more difficult than simply painting the resin on the plywood, which has worked well for others, but I wanted to overbuild this tank for added insurance.


Properly wet out cloth shown at left. Note that it is transparent with no white areas.

After letting this coat cure for few hours I trimmed the cloth at the top. The resin was hardening up, but still pliable, making it easy to cut. After this coat it was smooth sailing, the hard part was over. I applied 4 additional coats, allowing at least 4 hours between coats. This happened over the span of a couple of days. I coated all inside surfaces, including where the glass would be glued and the exposed plies at the top.

It is recommended to scuff sand with 80 grit sandpaper between coats to ensure good adhesion if the resin has cured for longer than 8 hours. I was careful to not touch the resin with my bare hands to avoid getting oils on it and sacrificing adhesion.

I also added some home made bulkheads on the back for the 2 canister filters. Next time I will find some real bulkheads because the PVC parts are rather bulky.

Here you can see the oak 1x2 trim and the top. Most tanks over 33 gallons have a center brace. My idea was experimental, so I advise using a center brace in your design, unless you have some building or engineering experience, or you don't mind taking a risk.

Filled with water, the center of my tank bows out about 2mm. I have had this looked at by my nephew who recently earned an engineering degree and was reassured that this was fine. I used 3/8" glass which is good for up to 24" high. Since my water line is about 19" high, I have a little extra insurance.

When choosing glass for your tank, remember the general guideline that 1/4" glass is good for up to 18" in height, 3/8" is good for 24" high. I am not an engineer, therefore I am unsure about how this relates to the tank capacity, or how long the tank is. If you build a 300 gallon, 8 foot long tank that is 24" tall, you may want something thicker than 3/8" glass, that is why I recommend researching everything thoroughly before building your own tank.

The only thing left to do here is add the glass. I used 3/8" non tempered glass and had the edges arised to avoid cutting myself. I had the glass cut so that it goes all the way to the edges. Then I put down a fat bead of sealant and carefully laid the glass in the tank. I then put some weights on it and ran another bead around the edge. It was a messy task, but the tank does not leak a drop. I used about 3 tubes of silicone to seal the glass, the joints and around the top. Stock up on the silicone because you don't want to run out in the middle of this task. Estimate what you need and buy 3 times that amount. If you don't use it all you can always return it.

It took a lot of discipline, but I waited a full week after installing the glass to fill it with water. I then left it full of water for another week to make sure it would not leak and to help leech out anything toxic from the epoxy.


What would I do differently next time?

Build it BIGGER of course!! The tank seemed so big when I was moving it into the living room. But it seems to shrink every time I look at it. The next one's gonna be 8 feet long!

Another thing I would do is to lay the cloth and laminate the plywood before assembly. It would be a lot easier to do with all the boards laid out flat as the cloth can be a little cumbersome to work with. The fiberglass cloth is not necessary, but I know that is it stronger than epoxy alone. I don't have eggcrate in the bottom of my tank and I am confident that the heavy rocks will not puncture the epoxy.

I also would have tracked down some proper bulkheads. The PVC is bulky and difficult to hide.

Was it difficult to build?

It wasn't bad. Obviously you need some mechanical inclination to do this, but I would say if you have average building/mechanical skills it shouldn't be a very tough job. The key is to take your time, be patient, and do some thorough research on both building tanks and working with epoxy before you attempt the project.

Was it cheaper than a glass tank?

In the end the tank was about the same price as a comparably sized glass tank. It ended up being a little more expensive than I had originally anticipated. I could have built it for less money, but once I started building it money became less of a concern and the challenge of building a plywood tank that would last for years became more important. You really start saving when you start building bigger tanks (180 gallons +). In the end I have something that I am proud of and I actually like it more than a standard glass tank. □

 

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