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Polycarbonate is not as clear as acrylic or crystal styrene which is why it is not used in aquariums. There is a lot of distortion in the material. Like looking through glass covered with vaseline.

Also you need to use a thicker wall material to keep it from flexing and bowing. A 24" tall acrylic aquarium would need to be at least 1/2" thick.
 

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A windshield and an aquarium are different things.

I actually have experience with making polycarbonate aquariums. We have looked at making Minibows out of polycarbonate in the past. It never comes out as clear as crystal styrene and other than R&D samples we tested, it's never been used in production. The tanks look terrible. And those are small desktop tanks. I couldn't imagine how bad it would look on a thicker, big tank.

A frame will only reduce the bowing at the top. But the center of the panel will still move out quite a bit from the water pressure. A typical glass aquarium will bow out in the middle about half the thickness to the full thickness of the front panel. The glass doesn't stretch or get longer, it pulls it in from the sides. Silicone allows a flexible seam which allows for this. For acrylic and polycarbonate which are glued with a inflexible seam, the bowing of the panel actually causes the material to stretch. If the bow is more than the material can stretch the seam will pop or the panel will crack. Which is why you need to use a thicker panel to reduce the amount of bow and keep the sides from popping.

Believe me, If polycarbonate was a lower cost alternative to making acrylic (or glass) aquariums. We would have done it by now.
 

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Silicone can not be used to seal a polycarbonate or acrylic aquarium. Silicone does not bond to plastics as well as glass and the seal does not hold. You need to use a proper glue (Weld On #16 works well) to bond plastics.
 

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Plywood and fiberglass stock tanks used in aquaculture sometimes feature a polycarbonate or acrylic viewing panel. But you are right that it is the frame around the panel that is providing support. The panel is pushing against a silicone or epdm rubber gasket that is between the panel and the frame of the tank. Sometimes there are screws or bolts actually holding the panel in place. Since there is no seam that needs to be held together it will not leak or separate.

I have worked with several acrylic manufacturers and they will tell you straight up that silicone will not hold a plastic tank together. I would trust people in the industry with years of experience.
 
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