# Doubts about Plywood tank



## zhouluyi (Apr 18, 2008)

Hello folks,
I'm planning a plywood tank with a front window, like most of the plywood tanks. I'm just having some concerns on some specific points not fully explained or with conflicting opinions.

1) Order of assembling the tank
The sides and front should be screwed into the bottom or the bottom should be screwed into the sides and front? What is the reason for one or the other?

2) Frame size
How wide should be the frame edge that is holding the front glass, considering just the size that comes in contact with the glass. Like, in a 2 3/4" wide frame just 2" contacts the glass, the remaining is used to drill the frame to the body of the tank.

3) Type of braces
Wide center brace? Many thin braces evenly spaced? Perimeter braces? Why?

4) Sealing the corners and inside
I'm planning on making round corners with auto body filler, then giving the inside 1 coat of polyester resin, 1 coat of fiberglass/polyester, a 2nd coat of fiberglass/polyester in the corners for reinforcement, then 3 or 4 coats of epoxy paint. Is that Ok? What can be done better?

My tank will be 150 x 45 x 65 cm (60 x 18 x 26 in), about 400 liters, the plywood is 20mm (3/4") marine plywood, the glass will be 10mm (3/8"). For polyester, fiberglass, and autobody filler I will use the most common stuff as this is the only thing I can find around here (I live in Brazil), for the epoxy paint I will use a brazilian brand made for tanks and pools and approved for potable water (similar to Epoxide HS or Tank Clad HS).

Unfortunetly I can't find most specific stuff or materials around here. I had a lot of trouble to find the epoxy paint for tanks already, I tried to find epoxy resin with no luck. So I'm tied to use "generic" and common stuff.

Luiz Borges


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## Mcdaphnia (Dec 16, 2003)

zhouluyi said:


> Hello folks,
> I'm planning a plywood tank with a front window, like most of the plywood tanks. I'm just having some concerns on some specific points not fully explained or with conflicting opinions.
> 
> 1) Order of assembling the tank
> The sides and front should be screwed into the bottom or the bottom should be screwed into the sides and front? What is the reason for one or the other?


The tank should be assembled twice, once â€œdryâ€


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## Mcdaphnia (Dec 16, 2003)

I forgot to mention that if you are reinforcing the tank with fiberglass cloth, reinforce the top and inside edges of the bracing too. For some reason these are the most scratched up parts of a plywood tank. I'm sure I never set a heavy rock on top of the brace and then slide it into the water,  but to look at the edges of my plywood tanks, somebody must. :-?


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## zhouluyi (Apr 18, 2008)

Thanks *Mcdaphnia*, in my "research" about the subject it was one of your posts that gave me another look on how to assemble the tank, and it made a lot of sense.

Just some points that I got confused:
You said I should have a front and back braces (I was thinking on doing that). I didn't quite understood what is the cross and ends braces, and how to set it (I will be making the braces out of the same plywood).

From what I understand, the cross brace is the "center" brace that I mention below, right? And the end braces are the ones similar to the front and back braces but put at the ends, correct?
If so, they should be put bellow or above the front and back braces?

Also, is a cross/center brace really needed for a tank of this size? If so, does it need to wider than a 15 cm (6")?

About the pre-drilling, should I drill just the first board (to guide the screw), or the also the second board where the screw is set?

I got the bondo idea on some of the many DIY plywood on the web, it said that fiberglass would make a smoother curve this way being less prone to crack, I never worked with fiberglass so I don't have a clue.

I also wanted to fiberglass it because I'm worried that a rock might crack the paint and make a leak.

I really wanted to make the tank deeper, but my wife set some limits on the space available to me in relation to the furniture 

Thanks again for your help,
Luiz Borges


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## Mcdaphnia (Dec 16, 2003)

zhouluyi said:


> Thanks *Mcdaphnia*, in my "research" about the subject it was one of your posts that gave me another look on how to assemble the tank, and it made a lot of sense.
> 
> Just some points that I got confused:
> You said I should have a front and back braces (I was thinking on doing that). I didn't quite understood what is the cross and ends braces, and how to set it (I will be making the braces out of the same plywood).
> ...


You have the idea right about the braces. The end braces are just like the center brace, only set at the end. If you want, you can make one of them wider and put cutouts in it for a heater and outside power filter. on the end of the tank.

It is your preference whether to put the cross and end braces below or on top of the front and back braces. Although if you think you may some day use a hinged glass top, you will want to install the end and center braces below the front and back so that the glass top does not fold up and fall in. I like having the glass top a little recesed this way because water that gets caught under it returns to the tank instead of oozing down the outside of the tank, causing extra cleanup work and a panic if someone thinks the tank has a leak.


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## zhouluyi (Apr 18, 2008)

Thanks again for your help,
I don't think I will use a glass top, I thinking on a canopy with 2 or 3 40W (4') light tubes, I might put some glass plates to avoid jumpers, but I'm also considering putting an open mesh net top (2-3 mm of space between fibers), that way I wont have to worry about condensation, the light will get a bit diffused, and no jumpers will be able to pass through it, any thoughts on that?

I did a sketch of your thoughts, this is what you meant:

The top and bottom front frames are 3", the side frames are 2", all the perimeter braces are 1 3/8" and the center brace is 6".

Luiz Borges


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## IrkedCitizen (Apr 26, 2007)

Another way to consider doing it is like this. I am kind of a fan of this method. I have not yet made a plywood/glass tank but if and when I decide to do one I'd probably do it similar.

The main reason is you could build it in your garage or work area of choice and then take the finished pieces to the final place for the tank individually and then assemble it in place. This is a big deal if you make a giant tank like the one on the link that couldn't possibly fit through a door fully put together. Saving you from having to build it in the room where it is intended to go. Also, if you move you can take it with you instead of having to leave a giant tank in say your basement. But of course that only holds true for very large tanks.


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## Mcdaphnia (Dec 16, 2003)

IrkedCitizen said:


> Another way to consider doing it is like this. I am kind of a fan of this method. I have not yet made a plywood/glass tank but if and when I decide to do one I'd probably do it similar.
> 
> The main reason is you could build it in your garage or work area of choice and then take the finished pieces to the final place for the tank individually and then assemble it in place. This is a big deal if you make a giant tank like the one on the link that couldn't possibly fit through a door fully put together. Saving you from having to build it in the room where it is intended to go. Also, if you move you can take it with you instead of having to leave a giant tank in say your basement. But of course that only holds true for very large tanks.


 Irked, check out the followups on this design before you fall for it. Every connection you make from the tank to supporting beams is one more potential leak. My 520 did look superficially like this, but the tank was not connected to any of the walls supporting it, (and all the spaces between the studwork was filled with styrofoam insulation).

This tank IMO was not big enough to justify the modular construction. It was much simpler to knock a hole in the wall and move my 520, then repair the wall, than it would have been to take the tank apart like the Norwegian one. I've participated in a 3000 gallon tank built in a modular way, but not quite the Norway design, that was displayed at a Cleveland Aquarium Society annual fish show in the mid-70's.


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## IrkedCitizen (Apr 26, 2007)

Mcdaphnia said:


> IrkedCitizen said:
> 
> 
> > Another way to consider doing it is like this. I am kind of a fan of this method. I have not yet made a plywood/glass tank but if and when I decide to do one I'd probably do it similar.
> ...


I did read the follow ups that he posted. Your points are all valid points I suppose but if you build the tank in your basement it isn't really an option to cut a whole in the cement foundation of the house. If you wanted to take the tank with you, you would have to completely take it down.

I don't know I don't really see myself building a plywood tank any time soon anyway. I have no room for more tanks and all of my tanks are glass.


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## zhouluyi (Apr 18, 2008)

Here is my new design:


Here is the detail of the braces:


Tank Dimensions: 150x45x60 cm (59 x 17 3/4 x 25 1/2 in)
Widths:
Top frame: 8 cm (3 1/4")
Bottom & Side frames: 6 cm (2 1/4")
Front, Back & End braces: 4 cm (1 1/2")
Center braces: 6 cm (2 1/4")

All of those with 20mm (3/4") marine plywood. Internal border to hold the glass 4 cm (1 1/2") all around.

Is that design, ok? Any needed changes for improved security, ease of maintenance, aestethics?

How long should be the screws used? And which of those are better to be used, just to make sure and avoid name confusion:
391 (Softwood, MDF, Chipboard):








412/410 (Hardwood):









Thanks again,
Luiz Borges


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## Mcdaphnia (Dec 16, 2003)

zhouluyi said:


> Here is my new design:
> 
> 
> Here is the detail of the braces:
> ...


it is really hard to see the board detail in the drawings, but it looks like esthetically pleasing proportions.

I would expect the right screw length to be about 1.5", twice the thickness of the plywood, but there should be at least seven screw threads tying in to the second board. More than seven adds no improvent Either screw should work. there is something to be said about an unthreaded shank. It allows you to "draw" panels together. It is a bit stronger so you are less likely to lose any screw heads,


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## zhouluyi (Apr 18, 2008)

Again, thanks for your great help Mcdaphnia. 



Mcdaphnia said:


> it is really hard to see the board detail in the drawings, but it looks like esthetically pleasing proportions.


Did you see the full size image? Any part is of particular interest?

Luiz Borges


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## fishwolfe (Mar 27, 2005)

just posting to follow along.still planning a plywood tank sometime soon.good luck with yours. opcorn:


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## Mcdaphnia (Dec 16, 2003)

zhouluyi said:


> Again, thanks for your great help Mcdaphnia.
> 
> 
> 
> ...


I just now clicked on it! The ends are 3/4" too short and the bottom 1.5" too long. Having any screw heads under the tank creates a potential leak site. Don't feel bad. Notice that this tank construction website starts outs showing the bottom detail correctly, then switches in the rest of the drawings. 
http://www.sanitred.com/waterproofing-b ... dfountain/ 
You can only have it both ways on paper (or screen). If you have access to a 3-D drafting tool, it's a great idea to spend lots of time playing with it before a single board is cut, just to find inconsistencies like this one.

Remember not to attach the end and cross braces until the very last step, AFTER the glass has been installed. Whether the braces are glass or wood. they can be siliconed on instead of screwed on, which gives you the option to more easily replace the front glass should it someday become too scratched and foggy, or breaks. The local fish clubs here used to host plywood tank workshops, and there was always someone who forgot this, and had no way to install their front glass into the tank. Sometimes they could leave the end braces in place (a tank with lots of depth of field lets you angle the glass as you lower it), but had to chop out the cross braces in order to lower the glass into the tank. A little extra work, some wasted material, but mostly just that warm oops feeling.


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## IrkedCitizen (Apr 26, 2007)

fishwolfe said:


> just posting to follow along.still planning a plywood tank sometime soon.good luck with yours. opcorn:


Just as an FYI...below the quick reply window at the bottom of each thread there is a place that says "watch this topic for replies." if you click on that it will subscribe you to the thread so you don't have to actually post a reply in it and if you choose to not want to receive notices of a reply in the thread in the same spot it will say "stop watching this topic" you can then click that. :thumb:


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## zhouluyi (Apr 18, 2008)

Mcdaphnia said:


> I just now clicked on it! The ends are 3/4" too short and the bottom 1.5" too long. Having any screw heads under the tank creates a potential leak site. Don't feel bad.


Oooops, my bad, I actually design it having the ends being screwed to the bottom like you said earlier, I just forgot to correct that in the drawing. 

I remember what you said about screws in the bottom, and the force applied in each case. I just left that [wrong] line there, if i've posted a picture from the bottom, you would have seen the correct lines (joints) already done. :thumb:

EDIT: Here is it the bottom view, with the wrong line already erased.


Luiz Borges


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## mithesaint (Oct 31, 2006)

Mcdaphnia said:


> IrkedCitizen said:
> 
> 
> > Another way to consider doing it is like this. I am kind of a fan of this method. I have not yet made a plywood/glass tank but if and when I decide to do one I'd probably do it similar.
> ...


So the modular idea is a bad idea?? How big do you have to go to make it worthwhile?? I already have the glass for my DIY (87 x 37 x 3/4" and FREE!!!), and was planning on doing the modular approach because I know I won't be at my current house for more than a few years, and don't want to have to chainsaw my creation. It has to go into the basement due to weight, and the only way out is up the 34" wide stairs. Would I be better off just putting the glass into a frame to protect it, and building the rest after we move in a few years??


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## IrkedCitizen (Apr 26, 2007)

The modular way isn't horrible. You just have to use thicker plywood than that guy used and less vertical supports. I think mcdaphnia's biggest complaint is that the more screws used the more places a leak can occur. I would use fiberglass sheet on all of the pieces.


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## Mcdaphnia (Dec 16, 2003)

It's hard to set a certain size before modular tank is better than a solid tank. How often a person moves, and their preferences in homes has a big influence so it's hard to say that modular should be used when a tank is taller and deeper than a door or hallway or stairwell.

One thing I'd change from the mod design, is to greatly reduce the number of boards attached to the tank. Any support needed can be added to the stand and will be stronger that way because it is continuous from the floor to the top of the canopy. It will mean more parts, but that is good in this case (transport) because they will be lighter and smaller.


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