Why do aquariums have glass bottoms? - The Planted Tank Forum
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post #1 of 21 (permalink) Old 01-09-2011, 12:57 PM Thread Starter
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Question Why do aquariums have glass bottoms?

This is something that has bothered me for some time. The glass bottom seems to serve no propose in the look or style since no one can see them once covered. It seems to be a weakness more than anything. Can any body help me answer

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post #2 of 21 (permalink) Old 01-09-2011, 01:34 PM
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Originally Posted by Patriot100% View Post
This is something that has bothered me for some time. The glass bottom seems to serve no propose in the look or style since no one can see them once covered. It seems to be a weakness more than anything. Can any body help me answer
Because glass is cheaper then anything else they could use with the same or better strength.
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post #3 of 21 (permalink) Old 01-09-2011, 01:36 PM
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What else are you going to make them out of?
Plastic or resin would have to be thick or well engineered to not sag when the tank was lifted with some water in it. It could work OK for very small ones, but larger tanks it would be difficult. You would need support for plastic all along the bottom, like a table, not like some stands that just support the edges of a tank.
Metal is possible, but you would probably want it coated in epoxy or some such to make it unaffected by water. You could use stainless steel, but piece that is as strong and stiff as an equal size piece of tempered glass would be heavy and more expensive.
Plus, I wonder about different rates of thermal expansion for different materials.
Probably a metal reinforced resin would work, but again, it would take more engineering.

And finally, an aquarium maker is set up to work with glass; they have the tools and knowledge to make it work. And it does work, all the way up to the biggest commercially available sizes. It may seem like a possible weakness, but the bottom is not where most tanks fail. (It is the seams or from the front/back bowing if supports are removed.)
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post #4 of 21 (permalink) Old 01-09-2011, 01:41 PM
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old tanks used to use slate bottoms but i beleive the industry has steered away from that do to weight.
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post #5 of 21 (permalink) Old 01-09-2011, 01:44 PM
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Glass is a surprisingly very strong material. (I would know I was studying to become a Glass Materials Scientist/Engineer (changed to physics because glass is still just glass but that's beside the point). Tempered glass is VERY strong in one direction. Think of your car windshield. It's tempered to be able to protect you and when it breaks it's designed specially so it won't fracture into large, dangerous pieces.

Aquarium glass is also tempered. It can withstand lots of direct pressure without breaking. It's also relatively lightweight and cheap. Like some previous posts have said before me you could use a coated metal such as some shade of steel but it would be heavy, expensive, and need to be coated to prevent contamination/deterioration with the water. (even more expensive)

Plastic is currently not strong enough to be manufactured to an equivalent piece of glass unless you make the plastic very thick (unsightly, inefficient, expensive, unnecessary)

Glass is a fantastic option for them and that's why it's proven the test of time.


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post #6 of 21 (permalink) Old 01-09-2011, 03:34 PM
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Quote:
Originally Posted by tzen View Post
What else are you going to make them out of?
Originally they were made out of slate. Not sure when that fell out of favor, but I imagine glass is cheaper and easier for mass production.


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post #7 of 21 (permalink) Old 01-09-2011, 04:45 PM
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The slate bottoms also had an issue with deteriorating over time. Slate is a sedimentary rock and is in thin layers. Over a time period of many years, with the exposure to water, these layers can separate.

Acrylic plastic would be strong enough. There are acrylic aquariums available. Most of the large tanks you see in public aquariums are made of acrylic. With glass sides the issue is bonding the glass to plastic. You have two different materials that expand at different rates, most seals would fail.

Only some tanks are made of tempered glass. If fact, if you want to drill the tank to install an overflow, you have to make sure the tank is not tempered. Drilling tempered glass will cause it to break.
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post #8 of 21 (permalink) Old 01-09-2011, 04:55 PM
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The slate bottoms also had an issue with deteriorating over time. Slate is a sedimentary rock and is in thin layers. Over a time period of many years, with the exposure to water, these layers can separate.

Acrylic plastic would be strong enough. There are acrylic aquariums available. Most of the large tanks you see in public aquariums are made of acrylic. With glass sides the issue is bonding the glass to plastic. You have two different materials that expand at different rates, most seals would fail.

Only some tanks are made of tempered glass. If fact, if you want to drill the tank to install an overflow, you have to make sure the tank is not tempered. Drilling tempered glass will cause it to break.
Bingo!

The silicone sealant used in aquariums creates a very strong bond with glass, but not plastic. It's best to use the same materials because they will contort to environmental fluctuations at the same rate.

There are some sweet builds of plywood tanks with glass fronts on this forum. Wasserpest has an awesome 250 he built himself with only one pane of starphire glass. But for the average consumer, a glass box is probably what works best.

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post #9 of 21 (permalink) Old 01-09-2011, 05:13 PM
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The slate bottoms also had an issue with deteriorating over time. Slate is a sedimentary rock and is in thin layers. Over a time period of many years, with the exposure to water, these layers can separate.
Never even thought about that, but you're so right!


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post #10 of 21 (permalink) Old 01-09-2011, 06:26 PM
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Silicone sealant in the '70s killed off the slate bottomed tank. Before silicone sealant tanks were sealed with tar which isn't nearly as good. Silicone sealant won't bond with slate for very long so they started using glass which bonds with the sealant very well.
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post #11 of 21 (permalink) Old 07-01-2013, 05:35 PM
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Correction.

Quote:
Originally Posted by DaveK View Post
Slate is a sedimentary rock.
Slate is metamorphic, not sedimentary. Hard to believe that glass is the strongest and cheapest material they could think of, although, the thing about the silicone bond makes sense.
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post #12 of 21 (permalink) Old 07-01-2013, 06:03 PM
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Slate is metamorphic, not sedimentary. Hard to believe that glass is the strongest and cheapest material they could think of, although, the thing about the silicone bond makes sense.
what other materials do you have in mind?


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post #13 of 21 (permalink) Old 07-01-2013, 06:22 PM
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There are stronger lighter materials than glass, but you can't see through them Acrylic being the only real exception to that rule.


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post #14 of 21 (permalink) Old 07-01-2013, 06:46 PM
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Like others have said,
You can't seal glass to other materials without issues.
Slate is HEAVY. It's hard to get it not to leak.
Tar dries, hardens and chips.
The trim on tanks you see is mainly to keep the long sides from bowing, but otherwise pointless. I really don't know why the US doesn't do 'euro bracing' as a standard, I guess the US just wants to be different.
Not all tanks are tempered, many bottom glass is, but the walls aren't. [or maybe I have that backwards...? lol]
Glass is strong, clear, takes scratches better, light, glues well, etc.

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post #15 of 21 (permalink) Old 07-01-2013, 06:53 PM
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I really don't know why the US doesn't do 'euro bracing' as a standard, I guess the US just wants to be different.
.
cost.. people are cheap

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