Can be done and certainly feel free to try but I give some thought to projects of this type that I think about. If it is that easy, why do we not see it done more often?
Are you the first bold person to want to build a large tank cheaply?
Are you the first to think of concrete as a material for tanks?
Are you sure that you will be better than the others who have tried?
If so, throw down the money and go for it!
There may just be a "better mousetrap" in your future!
We probably haven't seen it done on a small scale (as discussed its done all the time on large scale) because:
1) Most people prefer the look of glass, you have to really want at least one side to look like concrete before this is even something worth pursuing
2) Even assuming this works perfectly, the build will be significantly more difficult then a glass tank build since you need to make a mold, mix the concrete, pour it into the mold, let it set, sand the concrete, seal the concrete, and let it cure all before you ever get to assemble it
3) This build is cost effective (assuming it works) only because I can buy premade tempered glass panels at a fraction of what custom cut annealed glass panels would cost, if it were not for that one issue there would be little to no cost savings
4) GFRC concrete has really only become popular in the public eye in the last 10 or so years, so there would have been less public awareness of this even being possible before that
5) With all of that taken into account we also would only hear about it if someone posted their build and or experience on a popular aquarium forum
So that is probably why we haven't seen more of these builds. Then again the whole thing could fail spectacularly at several different points. So maybe that is why? I really can't claim any degree of certainty in success at this point in time. I can say that I am very interested in people's experiences who have worked with gfrc concrete to create furniture or other similar objects and also interested in anyone's experience attaching glass to concrete. If folks haven't done either, I admit I approach their recommendations with a large grain of salt.
I can promise the people here only that 1) I plan to make some test boards and assuming that works to make this aquarium as laid out, and 2) That success or failure I will post my results.