Have never really given that any thought, but there is a lot of lime in the portland cement which is used to make hypertufa. Wouldn't that interfere with the ph of the water as it leached out? The peat moss is acidic and that would also break down. Whenever I had aquariums, I don't recall having seen structures in the shops made out of concrete. Maybe if lime and the peat moss could be neutralized, it wouldn't have any effect.

When we were doing some research with "pastacrete", we found out that the artificial reefs in the big aquariums are made from portland and agregates and pasta is used in producing the coral shapes. This is very tricky cause we also learned that the carbohydrates in the pasta nearly stops the curing process so you have to remove it by washing it out. I couldn't find the original link, but here is one to show you it can be done.

Jo

Hi Jerry, The ph will stabilize in your pieces after a good 28 day cure. So if you were to cure your pieces under water they'd be just fine to put in your aquaruim after a month. Go for it and good luck.

Some of these addresses might be useful.
Paws
http://www.reefsuk.com/Marine%20Aquariums/Articles/garf_zero_impact_aquarium.htm
http://www.scubaraven.com/diy_liverock.htm
http://www.aquaserve.com/articles/concrete_rock.htm
http://showcase.netins.net/web/reefpage/mmliverock/mmliverock1.html
http://www.petsforum.com/cis-fishnet/seascope/98SS1507.htm
http://www.reefs.org/library/talklog/t_miller_052498.html
http://www.garf.org/MPegs/AragocreteArch.html

I use portland and silica sand or aragonite/calcium based sand, crushed coral.

..btw you can also check www.reefcentral.com

How big will the set-up be? If it is a small home type aquarium, you might want to consider polymer clay. It is a plastic material that becomes inert once it is cured. It is baked in a home oven or toaster oven at a low temperature (about 275F) and is easy to work with. You should be able to find lots of additional information on it on Google.
Sounds like a fun project. Enjoy it.
spiderwoman