Plywood Tank Builders’ I need your help

[chance_11]

I am considering building a plywood tank in the 250 gallon range. I have some structural/materials questions. This is a long post, but anywhere you could help out would be appreciated. Attached (hopefully) are two pictures, one of the 135 where the tank is going, and the other is a quick drawing of the tank, stand, and overflows. Take a look at them to understand the questions below.


Here are my tank building questions:

I am going to use AC plywood. Does it have to be Marine grade/Outdoor grade? I’m thinking both of those mean the wood was pressure treated, and Marine grade is stronger with less internal voids.

The price Lowe’s gave me was $82/sheet for ¾ marine grade AB plywood (didn’t have AC) and $59.56 for 1/2, everyone else is paying around $50. Anyone know a good source near Dallas?

I’m guessing I need at least 1” plywood. What do you guys think? Could I go with ¾”, do I need to laminate two ½” sheets, or do I need two ¾ sheets laminated together?

For the glass, I think it needs to be 5/8 or possibly ¾. If I go with ¾ I will probably make the tank a few inches taller. What would you guys recommend? Tempered or plate? Can I use 5/8 tempered with a slightly taller tank?

Does the glass need to fill the front wall completely minus a ¼ inch gap? The reason I am asking, I want to have a 5-6 inch wood boarder on the bottom to cover up the DSB. Does the glass need to start at the bottom or can it start say 4 inches off the bottom of the tank? Any structural issues I’m not aware of here?

I want the Overflow to run down to the bottom of the tank with bulkheads drilled in bottom plywood I won’t have room behind the tank, and I want the tank to be as large as possible. Will Acrylic stick to the epoxy paint? I’m guessing you would put this in after painting and before putting silicone in all the joints? Anyone have experience here?

I am worried about scratching the epoxy, and plan to coat the inside of the tank with ¼ inch Plexiglas. For those of you that have done it, did you bond the pieces together or just set them against the wall/bottom? What did you do about the silicone joints? Is water getting behind the acrylic a concern?

Here are my Stand questions:

For the stand, I’m planning to use 1” square steel tubing. Looking at my stand design, I don’t have any cross braces along the middle 52 inches. I did this so the cabinet I’m building could also house a big screen TV. I plan to skin it with ¾ plywood on top to distribute the weight. Do you think the stand will hold? Does it need additional supports? I’m guessing the people who buy my house won’t want two huge fish tanks built into the wall. This way, it can be converted into a built in media center. Any input on the stand would be appreciated.



I’m putting the glass on two sides, and the overflow in on one side. This is just in case I decide to make this into a room divider of my next house. I can add another glass section to the back. (I say that now, but will probably just build an even bigger tank!) The dark blue will be two 1” returns, one to a couple spray bars along the back of the rock work, and the other pushing a lot of water across the tank. I will probably get an
Ampmaster 3000 to run the system. For the other three I was thinking about using 3-1” drains. Then I could add some power heads at the other end for wave action. Any flaws here? I have seen people just put bulkheads in the bottom of the tank for recirculation or return plumbing, but that scares me. Those bulkheads leak, and I’m draining the entire tank. Not worth the risk in my mind.

Thanks for reading the post, and I look forward to your comments. If I build this monster, I plan to track my progress on my website listed in my signature.

Thanks,
Sam

 

[mongo]

I'm no expert but here is my $.02 worth after building some good size plywood tanks.

I am going to use AC plywood. Does it have to be Marine grade/Outdoor grade? I’m thinking both of those mean the wood was pressure treated, and Marine grade is stronger with less internal voids.

I think marine grade plywood is a waste of money. If your plywood ever gets wet the tank leaks and you have bigger problems. I always use plain AC and if properly sealed it will never be in contact with water.

I’m guessing I need at least 1” plywood. What do you guys think? Could I go with ¾”, do I need to laminate two ½” sheets, or do I need two ¾ sheets laminated together?

Given the depth of your tank I think 3/4" would be fine. Obviously 1" would also work but may be overkill. Plywood is very strong and the pressure on the plywood is dispersed fairly well. If your woried about strength you could add in some braces across the bottom and/or ends.

For the glass, I think it needs to be 5/8 or possibly ¾. If I go with ¾ I will probably make the tank a few inches taller. What would you guys recommend? Tempered or plate? Can I use 5/8 tempered with a slightly taller tank?

I use acrylic so I don't know much about glass applications here. I looked into glass and if I remember correctely you want regular plate glass for thicker pieces. Thick tempered glass starts to lose clarity and get very expensive. You might at least look into acrylic, but it will scratch.

Does the glass need to fill the front wall completely minus a ¼ inch gap? The reason I am asking, I want to have a 5-6 inch wood boarder on the bottom to cover up the DSB. Does the glass need to start at the bottom or can it start say 4 inches off the bottom of the tank? Any structural issues I’m not aware of here?

I think you want something supporting the glass here.

I want the Overflow to run down to the bottom of the tank with bulkheads drilled in bottom plywood I won’t have room behind the tank, and I want the tank to be as large as possible. Will Acrylic stick to the epoxy paint? I’m guessing you would put this in after painting and before putting silicone in all the joints? Anyone have experience here?

The acrylic itself won't stick to the epoxy paint and silicon won't stick well to the acrylic. I can tell you the silicon will stick to the epoxy no problem. Why not build the overflow box out of plywood coverd on all sides by epoxy? Unless you need it to be clear for some reason...

I am worried about scratching the epoxy, and plan to coat the inside of the tank with ¼ inch Plexiglas. For those of you that have done it, did you bond the pieces together or just set them against the wall/bottom? What did you do about the silicone joints? Is water getting behind the acrylic a concern?

The epoxy is very durable. Ive poured gravel in, dropped rocks in and had no problems. I put on 4-5 coats of the stuff, the reccomend 2-3. Putting plexiglass on the inside is not a bad idea if you want extra protection but it may look bad after awhile. You will have stagnent water back there that's going to grow stuff that's going to be hard to remove. You could do this no problem on the bottom. To silicon the wood-wood joints I sealed it like any other glass tank.

 

[chance_11]

Mongo,

Thanks for the reply. Using wood for the overflow is a great idea. I don't know why I was stuck on using acrylic.

The concern with plexiglass over the epoxy is exactly what I am worried about. Anybody try this? do you get alge or have problems with water getting behind the acryllic?

Anyone else agree or disagree on 3/4 regular AC plywood and 5/8 plate glass? If I do go with 3/4 plywood, I will definately put some bracing inside along the bottom and top.

Thanks for replying,

Sam

 

[viking]

Hi,
I would use the 1'' ply but thats just me. I like strong.
Agree with Mongo no need for marine ply.. we hope its never going to get wet.
As for glass i would think that 1/2 inch would be fine but again 5/8 you can be sure and never have a worry about it.

I did my overflow in wood too. Another advantage here is it can be a used as a brace too. Very strong.

http://www.fishyfrolics.com/viking/plywood_tank.html

Cheers,
Viking.

 

[mongo]

The wood is cheap so if there is any question you might as well go with 1" instead of 3/4". If you want to protect the epoxy you could set heavy items on eggcrate. I recently got a big lace rock boulder that weighed a ton so I put some eggcrate down, cover it with sand, and then set the heavy rock on top. The eggcrate will help distribute the weight.

 

[Ken04]

Here are my tank building questions:

I am going to use AC plywood. Does it have to be Marine grade/Outdoor grade? I’m thinking both of those mean the wood was pressure treated, and Marine grade is stronger with less internal voids.


Thanks for reading the post, and I look forward to your comments. If I build this monster, I plan to track my progress on my website listed in my signature.

Thanks,
Sam[/quote]

Well there is a BIG difference in plywoods, so be careful. Marine plywood has an exterior glue which all exterior plywoods do, but the inside, the 'core' is what makes it different. Marine plywood is laid up in such a way that there are no core voids, which you'd see when cutting a piece if regular ext plywood. If you could find some Canadian A/C ext plywood that would be better than U.S. A?C, cause the Canucks 'core weld' their product. It isn't more than a few bucks so why not be safe ? And personally I'd go with 1" Marine grade, it's much, much stronger. Look at the ratings on plywood, they'll tell you how much you can span on a building with wall span first, and the the vertical or roof span next. IE; 1/2" 5 ply ACX is rated at say 24/16. And 3/4 isn't very high in that rating. Think about water which weighs 9 pounds per gallon. Ken

 

[dale nilson]

If in doubt about what type of plywood, look into MDO. It's made for billboards and other outdoor applications exposed to weather, cheaper than marine grade but better than AC exterior. It is already epoxy impregnated on at least one surface.

for some insight into a large plywood tank, try http://www.aquarium.net/0797/0797_6.shtml

 

[FrankK]

The plywood tank I'm building consists of 23/36-inch regular type AC plywood with 2x4's along every joint. Interior dimensions are 90L x 30W x 29H. It's fastened with decking screws and "Gorilla" glue and coated with 4 coats of ICI/Devoe 233H NSF-rated epoxy paint. I'm going with 1/2-inch thick untempered glass - hopefully soon to be delivered.

I'm working now on my fish room with power panel, blowers, stand, lighting, etc..

Had considered use of 316-alloy stainless steel all-thread for bow-bracing but figure it to unnecessary with all my 2x4 bracing.

Here's a link to a long Reef Central thread discussing mine and others' projects: http://www.reefcentral.com/vbulletin/sh ... adid=87733

 

[kycoralhead]

I have built several tanks out of plywood, but never a really long large tank. But I have learned some lessons. First, the way the tank is constructed at the top is very important. You guys are talking about long pieces of glass and tall- 30". This is like a standard 220 gal tank that when I filled up, bowed out at the top about 5/8". Just glass, but if you use plywood you will have the majority of the stress at the top. I used a solid piece of plywood, cut the holes in it for the glass tops, put about 3" all around and screwed and glued the top to the sides. You have to calculate how large the center brace has to be to allow the glass in after everything is built. To do this the tank has to be wide and not as tall. But this worked great. I fiberglassed all the wood, including the grooves for the glass tops to fit into. I mixed black coloring agent with the fiberglass resin and used fiberglass cloth at every corner and joint. I cut out the front and sides the same way with a 3" ridge all the way around each edge. You know the largest tank I have built this way is a 120 gallon, but I am wondering if the cost consideration is worth the effort. The cost of all the materials is going to be fairly steep. And not wanted to put the damper on but I know a person that built a tank and it came apart in the family room and caused a large rift in the relationship. It is fun to do, but I remember the first one I built, I checked for leaks every day for the first month. Use several coats of resin, use waterproof glue, acetone to degrease anything you silicone and be careful a 5/8" piece of glass is hard to handle. I am working on afish room, maybe I will get back in the building mode and make one too.

 

[dale nilson]

I've been looking into the plywood question more thouroughly, as I plan a plywood tank eventually, too. There is a large plywood mill in this area, so have good input from employees there.

The main difference between marine grade and exterior grade is the interior laminates. As has already been pointed out, exterior grade has voids, marine does not. The big difference, though, is the type of wood used. American and Canadian marine grades used either Douglas Fir or Western Larch (identical, quality-wise) throughout; exterior grades use cheaper, less rot-resistant woods in the interior. This, coupled with the voids, lets condensation wreck havoc easier. Both grades use the same glue. It's not "pressure treated" in the same sense as pressure-treated lumber for rot-resistance, which has a chemical applied in the pressurization procedure. All plywood is glued under pressure.

There are high quality mahogany marine plys from foreign supplyers. The big diffence for boatbuilders is that they don't surface check nearly so bad, something epoxy and paint negates.

MDO is a marine plywood, but with a paper/epoxy surface on one side, and is not finished as smoothly, hence the lower cost.

 

[FrankK]

I agree that there's a lot of strain on top of the view panel.. That's why I reinforced mine with a couple 2x4's.

But I don't think it's necessary to use marine grade plywood. You just need something strong and of good quality like type AC plywood. Need to make sure your joints are strong (hence my reinforcement with 2x4's at every joint) and need to make sure it's totally encapsulated with epoxy paint inside and out.

This encapsulation is what's critical... If your encapsulation fails, I think your tank will fail regardless of what type of plywood you use. I used 4 coats of the Devoe 233H "Bar-Rust" potable water grade epoxy paint for this purpose.

Have a good one!

 

[FrankK]

Oops! Double Post.