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I topped up the tank this afternoon with around 12 litres of water. When I placed the glass lids back on top, they almost fell in because the gap where the lid sits is now so much bigger. I then placed the plastic tops over that and they almost fell in as well. 

Yesterday these were perfectly flush, but now I measured 6mm gap between the middle of the lid and the front panel! I can push it back to where it was with a fair amount of force, but then it immediately flexes back as soon as I take my weight off it.

Is this normal? Should I be asking for a replacement tank?

flex.jpg

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Glass is somewhat flexible which is why larger tanks typically have bracing  in the middle of the tank. That black plastic with the annoying bar in the middle is designed to prevent the glass from flexing. You talk about glass lids and a plastic top. If you removed the plastic frame from the top of the tank or anything structural (a piece of glass for example) that was glued in the middle of the tank, then you're letting the tank flex and that's not good. It will lead to a much bigger problem than the top not fitting. In your photo there's what looks like silicone residue near the handle on the top edge of the glass. If you removed a structural member that was glued in there, that's why your tank is now flexing. Glass is expensive and thicker glass is much more expensive, so tank makers tend to use as thin a glass as possible and then use braces for extra support. If you remove the braces you weaken the tank and the glass will flex and ultimately fail completely.

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Tank designs account for bowing. The larger the volume, the more flex you're likely to notice. A bit of Googling will show lots of people with the same question you have, and lots of answers just like mine. My 40B flexes quite a bit in the center, but it's a large volume without any center bracing. Both 20s and 29s show a bit of flex in the center as well. I'm not sure I'd keep trying to press it back, as the pressure you're putting on the glass is uneven compared to the pressure exerted by the water.

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7 hours ago, Chris45 said:

I topped up the tank this afternoon with around 12 litres of water. When I placed the glass lids back on top, they almost fell in because the gap where the lid sits is now so much bigger. I then placed the plastic tops over that and they almost fell in as well. 

Yesterday these were perfectly flush, but now I measured 6mm gap between the middle of the lid and the front panel! I can push it back to where it was with a fair amount of force, but then it immediately flexes back as soon as I take my weight off it.

Is this normal? Should I be asking for a replacement tank?

flex.jpg

I don't think that's normal and I would be looking for a different tank. JMHO.

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39 minutes ago, Schwack said:

Tank designs account for bowing. The larger the volume, the more flex you're likely to notice. A bit of Googling will show lots of people with the same question you have, and lots of answers just like mine. My 40B flexes quite a bit in the center, but it's a large volume without any center bracing. Both 20s and 29s show a bit of flex in the center as well. I'm not sure I'd keep trying to press it back, as the pressure you're putting on the glass is uneven compared to the pressure exerted by the water.

Thanks this does put me somewhat at ease

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5 hours ago, Daniel said:

I took the center brace out of 125 gallon tank once, and after that the front panel would flex out about 1 inch when full with water... it never actually broke.

This blows my mind. Would the glass have done that if it were thicker? Did it not, in some way, pull on the seams at the corners? How long did it last that way? 

Did the whole thing take place in an alternate universe with different laws of physics?! 🤣

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Ok, so two things. 

One, the 90 degree ledge was providing a stiffening brace and due to the crack that is gone. 

Two, glass can and does bow a LOT without breaking, but as your tank was designed with the brace, it is safer with the brace.

So on to repair. 

If you can, drain the tank completely. Sister on a new bracing piece of glass to the underside of the ledge with a ton of silicone. Look up sistering a cracked 2x4 if you don't have a clear picture of what that means. The idea is to leave the cracked brace in place and strengthen it with silicone and uncracked glass. Allow it to cure the full length of time before refilling completely.

If draining the tank completely is beyond you, you can possibly get away with draining 50-75%--enough to get the glass rim dry, and allow it to stop bowing until the silicone cures.

If, when you refill the tank, bowing doesn't happen, you are in good shape. If it still bows, you need to think about a replacement tank.

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@Brandy beat me to it, but that top ledge does a lot more than just hold the lids in place, the moment of area it provides adds a lot of stiffness to prevent bowing. Think of it like bowing an I beam, then how much easier it would be to bend if it was just a straight piece of steel.

Because the top strap is still in tact in the rest of the panel, there will be a massive step up in stress in the element of the glass where the strap is broken. I haven't done deformable body mechanics in a good while but there will probably be all sorts of shearing forces and whatnot introduced because of this.

 

I made a quick little diagram to visualise if that helps.

181926636_885186178696396_5137701371565385791_n.jpg.e13eecdd95a1f03e33cd5096ed02cf3a.jpg

Definitely repair or replace I'd say.

 

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1 hour ago, Chris45 said:

Thanks for all the input - I will continue to monitor for now and make some enquiries about repairs

I'd recommend also dropping the water level in the mean time if that's feasbile with your tank. The total force on the glass is proportional to the square of the water height, so for example if you remove a quarter of the water you'll have almost halved the force (down to 56.25% from full) on the glass, as well as lowering the centre of pressure from a third of the way up the tank to a quarter of the way up the panel.

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