View Full Version : Cinderblock support inside the reef

08/20/2017, 01:52 PM
I'm building a large reef and need to support a glass "table" inside the tank underwater. It's 4' x 2'. Over the table, there will be a large vertical acrylic tank ~ 180gals ~ 1500lbs.

The table must be 3/4" glass - let's leave it at that.

The floor of the tank is plywood and epoxy.

I originally was going to build the table on glass legs but decided against it.

So I think cinderblocks will work best. They can handle the weight.

My concern is the acrylic tank sitting on the glass (underwater) sitting on the cinderblocks. This will sandwich the glass between acrylic over it and concrete under it submerged. The acrylic is softer, so I'm ok there (I think). The concrete glass interface concerns me.

Add a little sand during maintenance (no matter how hard I try), and I see pinpoint pressure points crushed by 1500lbs. The average pressure ~ 1.5 psi (over the 2' x 4'). The cinderblock surfaces are also a little rough.

I could :

1. Do nothing. It's not a big deal.
2. Coat the cinderblock tops with a thin layer of epoxy.
3. Put a dab of closed cell foam (or thin sheet) between the cinderblocks and glass.
4. """???"""

08/20/2017, 03:07 PM
I'm not sure how high you need to raise this, but glass blocks may be another solution, and could be siliconed in place.

08/20/2017, 05:07 PM
Are glass blocks structural? Can they carry the weight?

08/21/2017, 10:00 AM
They are pretty strong, I'm not sure what your total weight is but it just seems more fitting to me and cleaner overall.
I've seen huge walls several stories tall, and while there is a metal channel between them it's not a beefy metal channel at all so I'm sure they can bear a pretty good load.
Take a look at them, maybe search for what they can carry

08/21/2017, 10:03 AM
Total weight is 1500lbs ~ .75 tons. It looks like these glass blocks are cosmetic and have a ridge that would push against the plate glass. I couldn't find any completely flat.

I think I'll use the cinderblocks with a thin silicone glue between them and the glass. This will make them permanent fixtures but I don't plan on removing the glass later and removing the blocks later would be painful anyway.

Any concern with the acrylic on glass interface? Even if a few grains of sand do get trapped between them, I think they would embed in the soft plastic vs. cause a fracture of the 3/4" glass below. But still, would it make sense to add a thin layer of rubber or foam?

08/21/2017, 07:26 PM
Will the cinder blocks be visible?

08/21/2017, 07:31 PM
Not usually. There are covers over the dark zone, but you can open the cover.