Diy plywood tank. Sealing ideas and physics

[Jon_DK]

Hi all,
I know this topic has been up a 1000 times. I am struggling to settle on a build plan for my final large aquarium. Aquarium will be rectangular and approximately 2500 litre with glas front window. Structural strength will be secured with plywood. Nothing new here.
The sealing method is what I can't figure out. We all know that wood is not dimensional stable. Especially among the joints the can be serious deformation. The usual way to overcome this issue is to lock the wood by using glasfiber in either polyester or epoxy. This mostly seems to work, but I'm not sure for how long. I also don't like the polyester styrene foams as i will need to work on this indoor where my family lives. Epoxy might be a better option and comes in forms that doesn't smell as bad. Both epoxy and polyester resin becomes very brittle as time goes. Definitely not able to follow the movements of the wood. Instead they strive to prevent the wood sheets from moving.
I think that it would be better to acknowledge that the wood is not very stable, and seal it with something that can adapt to the dimensional changes over time. A pond liner Is a good example on such a material. Or those liquid rubber bands. One concern however with the liquid rubber is that if it adheres well to the plywood, it will have to deal with dimensional changes along plywood joints on a very small distance. This requires the rubber to expand locally maybe as much as 100%. Also both the rubber and the pond liner makes front glass sealing more difficult. Silicon is preferred to seal the front glas, but this adheres poorly to rubber.
So too me, the plywood tank builds described around the internet, though successful, is based on materials that does not seem optimal. A successful build strongly depends on craftsmanship, experience and maybe a bit of luck.
So i was thinking, glass and silicon goes well together. The main problem for me is that in very large tanks the glass thickness and price goes up way too high. Thin glass could be used if it was somehow supported over the entire area. So I've idea that came to my mind, is a perfectly rigid plywood box, lined with sheets of glass and with silicone to seal the joints. The joints should be wide enough for the silicone beam to easily absorb the plywood movements.
Between the plywood sheets and glass sheets a thin piece of styrofoam or rubber sheet could be placed to absorb small irregularities in the wood. When the plywood in such a design moves, this movement should be handled by the elastic silicone beam.
A concern could be, if the plywood areas are not perfectly flat. This would lead to tension in the glass, that could brake. So what if all sheets of glass is replaced with something that:
Silicon can stick to, is a bit flexible, is watertight...
Maybe aluminum sheet metal would work. Maybe some kind of pre fabricated composite board...
What do you think? Am i just way too pessimistic? Do you have any suggestions on liner sheets that have the properties needed?
I really would appreciate not to go the epoxy or glasfiber polyester way. Also pond shield is not easy to get in Denmark. Maybe you know somebody discussing this already. Thanks and best regards, Jon

 

[fishdance]

Yes, this has been discussed many times in many places. Unfortunately this forum is no longer the place for bigger tank builds and technology has improved so much that larger tanks become less of a problem.

What water depth is your tank? Under 75cm, it's easier to build all glass.

Glass technology has also improved. Tempered strength, laminate types, construction quality and consistency. Much cheaper and better than ten years ago.

Standard float glass can also be strengthened by design and steel frame encasement but safety always needs to be prioritised.

You have worked out wood is not very suitable for aquariums. Why use wood at all?

 

[Backfromthedead]

Yes, this has been discussed many times in many places. Unfortunately this forum is no longer the place for bigger tank builds and technology has improved so much that larger tanks become less of a problem.

What water depth is your tank? Under 75cm, it's easier to build all glass.

Glass technology has also improved. Tempered strength, laminate types, construction quality and consistency. Much cheaper and better than ten years ago.

Standard float glass can also be strengthened by design and steel frame encasement but safety always needs to be prioritised.

You have worked out wood is not very suitable for aquariums. Why use wood at all?

Lol so where is "the place for bigger tank builds"? Please, lead us to the promised land o great one.

To the OP,

What are the prospective dimensions thereabouts of your 2500 liter tank? This could make all the difference.

Besides that, I would suggest to look into glass tank designs with welded steel or aluminum frames. With a tank that large, made predominantly of glass and a height over 24", you are not going to avoid very costly 3/4"+ glass, even with a reinforcing welded frame.

I think you could make something like a 12'Lx4'Wx2'H tank, 600-700 gallons, out of all 1/2" glass, with a double layered "floating bottom" design, so long as it was designed and built with an effective top brace.

I have never built anything near that size (Shame on me I guess) so cant speak definitively. Best of luck.

 

[jjohnwm]

...What do you think? Am i just way too pessimistic?...

To be honest, I kinda sorta think you are!

Is wood perfectly stable? No...but nothing is. As long as the construction is done carefully and painstakingly, I think that 3/4-inch plywood is quite sufficiently stable for this use; you are building a simple box to hold water, not a lunar habitat or a research station for the bottom of the Marianas Trench. For tanks up to 24-inches deep, no external bracing is necessary, aside from all around the top. I've never built deeper than 2 feet, but plenty of folks have and have seemingly done well with simple 2x4 braces attached on end, like studs in a wall.

I read constantly about flexing causing leaks in an epoxy-sealed interior, and the litany always states that this is "obviously" happening along interior seams where "micro-fractures" form. If these mysterious micro-fractures are really forming along the corners, a simple internal bead of silicone, applied identically to the same bead in an all-glass tank, would solve any potential problems.

But...a tank that is glued and screwed together into a strong box, with bracing all around the top edge and a front panel made from a single piece of plywood with a window opening cut out of it...is exceptionally strong and stable. If there is any flexing, I suspect that it would be in the centre of the panels, caused when "drumming" occurs during water changes and filling/emptying. I've worried about this a bit over the years, but I've built a bunch of plywood tanks sealed with various types of epoxy and the very few leaks I have experienced were around the periphery of the front glass, never in areas of wood only. If the drumming were a bigger concern, perhaps in a deeper tank, the additional 2x4 exterior bracing would seem to be the solution; I don't know, I've never built really deep tanks.

F fishdance states that for smaller (under 24 inches tall) tanks, glass is easier to use. Maybe for some folks, although I can't see how. A plywood tank can be built at your own pace, taking as much or as little time as you wish. With all glass, there is always a time crunch once you start working, due to the limited working time of silicone. The larger the tank...the longer the seams and the heavier the glass...but you don't get any more time. It's not hard, per se...but it sure isn't easier than the leisurely construction techniques associated with the use of wood. And an all-glass tank is always subject to the possibility of a catastrophic failure if a seam lets go. Being careful during construction, and especially when arranging stands and support, reduces the risk...but it's ever-present. I've had a commercial tank, from a reputable builder, let go on me...trust me, you never forget that experience. A plywood tank might leak if you aren't careful in construction, but an extreme disaster like a split seam is virtually impossible.

Other materials are apparently in use today, and perhaps some of them would be preferable if you have no experience with...and thus no bias toward...building with wood. And, of course, some folks like to complicate things just for the sake of complicating things, so playing around with new materials and methods has its own appeal for them. I don't doubt that at least some of these ideas are as good as, or better than, building with wood. Don't know, don't feel the need to re-invent the wheel.

Personally, I think that your notion of building the tank out of wood, and then lining the inside with thin glass or other materials, sounds like a way to make the job much harder, more expensive, more "experimental", with far more potential pitfalls...and no benefits at all! But that's the opinion of a dinosaur; I'm sure others will find it worthwhile.

Whatever you decide, good luck with your choice and I hope you keep us updated here as you go.

 

[wannadivesteve]

I'm leery about lining any plywood tank with thin glass in the hopes of waterproofing it. Years ago I had an aquarium in which the manufacturer had added a mirrored glass inside the back wall. Front and back wall were the same glass thickness, but within a couple years the thinner mirrored glass cracked vertically through the entire middle. Front and back wall were enough to do the job, but obviously there was some flex in it and that cracked the non-structural glass (the mirror, which was about one third to half as thick as the back plate). It was still a structurally viable aquarium in that it held water, but the water was now obviously finding its way through the crack and into between the mirror and the back plate as the mirror coating was getting soaked. Ended up having to take it apart and removing the mirror and back then had a kind of ugly looking rebuild.

There is no perfect material, but epoxy seems to be tried and true. By the way, I think the poly resin is toxic or at least there have been a lot of speculation that it is. Could be just a curing issue. I did know one guy who had a 1000 gallon or so plywood tank built that had poly resin for the most of the build to save some money and then a couple/few coats of epoxy over that. Don't know if it was a curing issue or if the epoxy had enough micro-fractures that the water got through to the poly resin, but he never could keep fish in that tank alive.

 

[Jon_DK]

Hi all,
First I need to thank your all for taking your time to read my (long) add, and share your thoughts and experience. Each reply is highly appreciated. My tank dimensions will be 240 x 120 x 85 cm (Length, width, heigth). The front window transparent area will be 200 x 70.
I am heading towards a plywood box. All glass design could work, but I feel more confident in woodwork. The sealing 1. choice i know, is epoxy. But some concern remains with this product to me. I have to build the tank in my home. My personal concerns are:
- Proper wetting og glass fiber matts (epoxy seems much thicker compared to poly resin)
- Availability in Denmark
- Fast curring time
- Sanding dust all over the place (potentially toxic if mix ratio is not perfect)
- Brittle over time and prone to cracks
The pros list is long and well known

My thought of sealing (thin) panels with silicone beams inside a structural box, I agree might end up more complex with higher risk of potential leakage. So doing yet another internet browsing stumbled across a company doing custom welded pond liners (butyl rubber). It always made sense to me to use flexible pond liners in combination with wood, but all the folding issues and glass window sealing issue previously got me away from this method. I have seen a number of videos on youtube where windows are installed in raised ponds. This is done by both DIY and pond companies.
Some builders seals the liner to the back of the frontwindow using some kind of butyl rubber sealant. This way makes constructionvery easy, but the sealant adhesion to glass and liner needs to be trustworthy. The other way, which i think is the standard way, seems better to me. Here the liner is sandwiched between the front window and the plywood (also in a flexible sealant). This way the pressure on the front window effectively compresses the sealing beam onto the liner. This way basicly corresponds to the standard plywood/epoxy method, just replacing the epoxy with a pond liner.
The customizable pond liner thickness is 1mm. Very flexible and can handle impressive amount of impact. It is UV resistant (to a good enough extend).

The difference between a pond liner and those liquid rubber builds i think, is that the pond liner doesn't stick to the plywood, but can move around to some extent. I believe this means that wood deformation will cause less stress in the rubber membrane. I never quite understood why plywood/liquid rubber builders cared so much about rubber adhersion to wood. I think that no adhession is better. Without knowing for sure i also think that a pre-fabricated pond liner is made from 2-component stuff in a highly controlled envirenment, whereas liquid rubber is often 1-compoent and thickness is maybe difficult to control. (I am just guessing here).

I will post some pictures showing my plans soon.

Thanks again and best regards, Jon

 

[M1A1]

Since you're focusing on it quite a bit, how much do you expect this tank to actually 'move around'? What are the factors that would cause it or its components to flex or move?

 

[Jon_DK]

I am thinking on just the plywood itself. I found these data from a danish plywood company:

"Moisture-related dimensional changes
Softwood plywood will shrink and swell in both length and width corresponding to about 0.15 ‰ (mm / m) by 1% change in the moisture content of the board"

This means that a plywood board measuring 240cm exposed to a humidity change of 10% will "move":
> 0.00015*2400*10
ans = 3.6mm
As long as all plywood boards srinks og swells the same (humidity level is equal for all) there should be no problem. This would be the normal case i guess, but i can't be sure. I can acclimate the boards in the room before assembly and i could also measure the humidity. I guess higher density plywood with more layers are better also... But several mm dimensional change just sounds alarming when combined with sealing agent that is not flexible. The fact that thousands of succesful builds seems to last for decades with epoxy, however proves that worst case theory and practical use gives different results. This is what matters when it comes to judge if epoxy/plywood is a reliable solution. So when i continue to look for an alternative it is mainly due to
- pond amor availabitlity in denmark
- Lacking craftmanship working with epoxy and glass fiber
- Epoxy sawdust where my family lives
Regarding the dimensional change due to humidity, i think this is mostly a potential problem if the wood is not equally acclimated when assembly takes place.
Best regards, Jon

 

[jjohnwm]

That specification is for softwood plywood; you can also use hardwood plywood, such as birch, which would shrink/expand less. And the ambient humidity is not the same as the moisture content of the wood; a 10% change in humidity does not necessarily equate to the same drop in moisture content within the wood. I suspect that you will be painstakingly sealing not only the inside but also the outside of the wood, as is commonly suggested nowadays. If you allow the wood to dry and acclimate at the site...i.e. sadly, in your house!...and then seal it after construction you will not have problems, even with softwood plywood.

You should probably sit down before you read this part. None of my DIY plywood tanks have any wood finish applied to the exterior surfaces. They are plain bare wood. If any water somehow gets into or onto the wood, the bare exterior allows it to dry by evaporation very quickly, rather than trapping the water within the wood. If I were to make another plywood display tank, it would be sheathed on the exterior in wood paneling, grasscloth or some other breathable decorative layer lightly held to the plywood to maintain this breathability. Despite the fact that most of these tanks lived in non-climate-controlled rooms, with fluctuating humidity, I didn't spare a thought to the expansion or contraction of the wood. I did occasionally fret a bit about pressure changes caused by dropping and rising the water level during water changes, specifically about the possibility of those changes flexing the wood and perhaps cracking the epoxy. But after a few decades of building tanks and having no problems, one tends to relax a bit.

If availability is an issue, bear in mind that you have many choices aside from Pond Armor. My last few builds were sealed with a product made by Interseal and I found it much more pleasant to use than Pond Armor. It goes on like quality paint, spreads easily and smoothly, and is low-VOC...although I still use it only during the summer months, all windows open and exhaust fans blasting...and who cares what the humidity was? The Interseal epoxy was available locally and was also much less expensive than Pond Armor; it comes in a much broader variety of colours, in case you make the same mistake I did and choose something other than black...

I also never worried about sawdust, since I didn't do any sanding of epoxy. All my wood is sanded in the garage before being brought inside, and most epoxies I have used allow a very broad window of time during which they can be recoated without the necessity of sanding. Carefully cleaning the wood with a tac cloth after sanding is essential.

Can't help you with fibreglass and resin. I have almost no experience with it, and none at all with using it in an aquarium build.

 

[Hatredy]

I don't know if it works but Google Semco liquid membrane. Check out the Youtube channel Mischief and Fins for the video title
DIY plywood aquarium - sealant options - epoxies and other things. My account is new so I can't post links.