Plywood Tank - 400+ - Corner Overflows? Please help!

kput · May 17, 2020

I am in the planning stages of a 96x36x30 plywood tank, and am struggling to decide on how I'd like the overflows. Almost every plywood build I've seen uses some sort of external overflow across the back of the tank.
Due to where I'm placing the tank, 36" is allllllmost too wide, so the extra 3-4" I would need the tank moved out for plumbing just isn't going to work behind the tank. I know I also have the option of doing the plumbing down the sides of the tank, but both sides will be visible, so I really just don't want to.

Realistically, I'd like to do a corner overflow, drilled through the bottom of the tank, but I have not seen it done and would rather not try something that can't be accomplished properly...
I've found a vendor on eBay that sells acrylic corner overflows, that are customizable in size. I figured I could have them made in the appropriate height, and silicone them in, just like a traditional overflow. This brings up a few questions, however, that I feel might need addressed on such a tall tank.
I will be using pond armor, so I won't have any issues with adhesion on the silicone.

Is there a specific thickness of acrylic needed so it won't bow / bend / break with all the water pressure against it? Or will having water in the overflow push against the opposite side and make that a non-issue?

What size should the overflows be? I'd like to be able to flow 1400+ GPH through the tank, if needed, and I haven't yet decided on plumbing sizes.. I'd imagine 2" drains would be about right?

Is there something I'm missing here? What other options might I have? Will this work? I'd really like to avoid making the tank smaller, I'm already somewhat regretting going with 30" height over 24~, but if I have to drop the width down to 30", I drop below 400g and that will just make me sad....

Thanks for your help!

AlexG · May 18, 2020

You can use a corner overflow. I would use a piece of glass or plywood to make the corner overflow box. Acrylic will not bond to silicone/RTV or pond armor and if you want to ensure it holds water back glass or plywood would be a better material to use. The hole though the bottom should accept a standard size bulkhead based on the amount of drain flow in GPH needed. 2" should work at it will flow ~3300gph with gravity, but you have to account for any additional plumbing that will slow the water down on its way to your sump. I put several coats of pond armor on the overflow holes to seal them in place. I personally avoid holes in tank bottoms as a leak cannot be easily/properly repaired without a tank drain. I do not use an external overflow box for my plywood tanks. I just have DIY overflows similar to an H2O overflow which just requires a bulkhead and a plumbed drain line. The drain line holes are only a few inches from the top of the tank so even if they had a catastrophic failure they could not be cause the tank to completely drain.

Drain Flow chart
https://flexpvc.com/Reference/WaterFlowBasedOnPipeSize.shtml

kput · May 18, 2020

AlexG said:
You can use a corner overflow. I would use a piece of glass or plywood to make the corner overflow box. Acrylic will not bond to silicone/RTV or pond armor and if you want to ensure it holds water back glass or plywood would be a better material to use. The hole though the bottom should accept a standard size bulkhead based on the amount of drain flow in GPH needed. 2" should work at it will flow ~3300gph with gravity, but you have to account for any additional plumbing that will slow the water down on its way to your sump. I put several coats of pond armor on the overflow holes to seal them in place. I personally avoid holes in tank bottoms as a leak cannot be easily/properly repaired without a tank drain. I do not use an external overflow box for my plywood tanks. I just have DIY overflows similar to an H2O overflow which just requires a bulkhead and a plumbed drain line. The drain line holes are only a few inches from the top of the tank so even if they had a catastrophic failure they could not be cause the tank to completely drain.

Drain Flow chart
https://flexpvc.com/Reference/WaterFlowBasedOnPipeSize.shtml

Thanks Alex, I was actually just about to head over and start reading your thread. I was planning on following DIYKings build to a tee on this, but I've seen it mentioned you had issues with the tank bowing following his bracing? I haven't made it to your thread yet, but it sounds like I might need to re-evaluate a few things. I'm having trouble tracking down the 5/8 or 3/4 glass needed to go 30" high, and if I have to use thicker bracing on the rear / plumbing in the rear i might have to take down the width to 30" over 36". This knocks me under the 400 gallons I was hoping for...

If I can find good quality plastic corner overflows, I might still try that route, I'm unsure at this point.

AlexG · May 18, 2020

Not sure who you contacted on glass but from my experience local glass shops were not helpful as they did not stock some of the thicker glass and the ones that did were insane with pricing. I even had one glass shop flat out refuse to work with 3/4" glass. I got my glass from glass cages. I would also suggest talking to tank manufactures or the LFS to see if they would be willing to sell glass panels cut to size with polished edges. For my tanks the 24" tall tank has been fine with normal bracing. Not sure how 30" would hold up without external bracing but I would likely do it if it was my tank. One suggestion I would make is for the top frame to add a layer of fiberglass to the exposed plywood edges on the top of the euro brace. I have one small section on my smaller tank where water got into the plywood along its cut edge even though it was coated in pond armor there must have been a small area that did not get good coverage. I contained the issue and have stopped it but I figured it would be a good FYI for you since you are in the building phase.

kput · May 18, 2020

AlexG said:
Not sure who you contacted on glass but from my experience local glass shops were not helpful as they did not stock some of the thicker glass and the ones that did were insane with pricing. I even had one glass shop flat out refuse to work with 3/4" glass. I got my glass from glass cages. I would also suggest talking to tank manufactures or the LFS to see if they would be willing to sell glass panels cut to size with polished edges. For my tanks the 24" tall tank has been fine with normal bracing. Not sure how 30" would hold up without external bracing but I would likely do it if it was my tank. One suggestion I would make is for the top frame to add a layer of fiberglass to the exposed plywood edges on the top of the euro brace. I have one small section on my smaller tank where water got into the plywood along its cut edge even though it was coated in pond armor there must have been a small area that did not get good coverage. I contained the issue and have stopped it but I figured it would be a good FYI for you since you are in the building phase.

I just hit up your thread, turns out I've already been through most of it.

I just reached out to GlassCages for a quote, but I'm thinking I might need to scale things down a bit unfortunately. I'm trying to keep the build itself affordable for the tank, and not being able to source the glass locally is going to drive things up. If I can find a way to add internal overflows, I can stick with my 36" width, but I should probably go shorter.

I might try going 27" high? This allows me to stick with 1/2 glass from a safety standpoint and hopefully the pressure difference from 24 to 27" isn't substantial enough that the bracing would fail. This would make the tank 96x36x27?

I saw on your original build you did quite a few more additional support pieces across the back and sides then DIYKing did? On his tank he had two vertical supports across the rear, at 78" across, thats giving it a support every 26". If I were to do a vertical plywood support every 18" or so, it would give me 5 total, and I could do two vertical on each side as well. In your opinion, would you think this is sufficient?

Thanks for the advice on your tank top.. I am having trouble picturing the area you are talking about, is there any chance you could post a pic of that area?
I think I might also mimic your use of glass to avoid shadowed areas, but I'd need to work out a way to use glass lids with that. Those will be a must-have for me on this build.

RocketEngineer · May 18, 2020

To your original question, the problem with corner overflows is their lack of surface skimming which can result in oil slicks in the center back of the tank. Personally, I would consider a center one to be a better choice, a trapezoid shape comes to mind, that gets you space for drains (go beananimal, trust me) and a return. JMO

kput · May 18, 2020

RocketEngineer said:
To your original question, the problem with corner overflows is their lack of surface skimming which can result in oil slicks in the center back of the tank. Personally, I would consider a center one to be a better choice, a trapezoid shape comes to mind, that gets you space for drains (go beananimal, trust me) and a return. JMO

This still runs me into the problem of ending up too wide.. This is the only overflow box I've found that will handle larger volumes of water (shows 2400GPH), but only uses 1.5" bulkheads. I was thinking I'd wind up with 2" plumbing, so will the 1.5 allow me as much flow as I'd like to be able to have..?
There's also the width, Exterior Box Dimensions: 16"L x 4"W x 6.25"H

This puts me with a 36" wide tank, 5" off the way. I have a dividing wall portion that sticks out 30", so that would put the tank a full 10" further out. Hard to describe without a picture, but I still just think it's going to be too wide for the space.. I'd have to cut my dimensions down to 30" wide.

96x30x27h. Hmmm..

WvAquatics · May 18, 2020

I think what rocket engineer means is do internal center overflow instead of corner. So this will not add to the size of tank. In a half hexagon shape. The you drill 3 holes in bottom for 2" drains. So your tank size stays the same size just moves for er overflows to the center.

kput · May 18, 2020

WvAquatics said:
I think what rocket engineer means is do internal center overflow instead of corner. So this will not add to the size of tank. In a half hexagon shape. The you drill 3 holes in bottom for 2" drains. So your tank size stays the same size just moves for er overflows to the center.

Ah, I understand. My dilemma now is finding plastic internal overflows. I can't use acrylic, and I'm not really seeing a lot of custom options out there unless someone has a reccomendation

Here's my width dilemma, this is 41" from the wall

RocketEngineer · May 18, 2020

Why can’t you use acrylic? I think it would be fairly easy to do two line bends in a jig. No need for teeth, just do a smooth edge. And you don’t need to go 2” on drains, 1.5 is plenty big enough especially with a bean setup.

kput · May 18, 2020

RocketEngineer said:
Why can’t you use acrylic? I think it would be fairly easy to do two line bends in a jig. No need for teeth, just do a smooth edge. And you don’t need to go 2” on drains, 1.5 is plenty big enough especially with a bean setup.

Acrylic won't silicone to the pond armor coating I'll be using, unless there's a solution I'm not aware of to make it stick

RocketEngineer · May 18, 2020

Then build it integral? You do want it big enough to reach down into anyways.

AlexG · May 21, 2020

kput said:
I just hit up your thread, turns out I've already been through most of it.

I just reached out to GlassCages for a quote, but I'm thinking I might need to scale things down a bit unfortunately. I'm trying to keep the build itself affordable for the tank, and not being able to source the glass locally is going to drive things up. If I can find a way to add internal overflows, I can stick with my 36" width, but I should probably go shorter.

I might try going 27" high? This allows me to stick with 1/2 glass from a safety standpoint and hopefully the pressure difference from 24 to 27" isn't substantial enough that the bracing would fail. This would make the tank 96x36x27?

I saw on your original build you did quite a few more additional support pieces across the back and sides then DIYKing did? On his tank he had two vertical supports across the rear, at 78" across, thats giving it a support every 26". If I were to do a vertical plywood support every 18" or so, it would give me 5 total, and I could do two vertical on each side as well. In your opinion, would you think this is sufficient?

Thanks for the advice on your tank top.. I am having trouble picturing the area you are talking about, is there any chance you could post a pic of that area?
I think I might also mimic your use of glass to avoid shadowed areas, but I'd need to work out a way to use glass lids with that. Those will be a must-have for me on this build.

Here is the picture of the top brace section had moisture get into the plywood. It has been stable for a few months now that it was resealed. Next step will be a repair.