Is it possible? oddly enough i had a dream about it. I know the pressure would be great at the apex, but with the right support and silicone seams....would it be do-able for a pico sized tank (2 gallons or less)?
I think you could build one, and a stand that would hold it it would look interesting, and with the substrate in there the water would be like a trapezoid which could be really cool I think. I think if I was to build such a thing I would investigate doing it out of acrylic instead of glass. and build a clear acyclic stand that supports the sides about 1/2 way up or so.
The more I think about it the more I'm sure you could manage a small tank that shape.
I can think of a couple ways to do it, and 5 gallons should be doable.
But aside from being cool, why do it? The bottom area for planting would be very small unless you had substrate filling up half of the pyramid. Did you have a really bushy plant that doesn't need much of the way in roots in mind?