i never thought of doing that for rocks. i will be setting up a 125 with all mbuna here soon and trying to figure out how to decorate it.
I actually have been doing it for quite some time, I first tried the styrofoam version but found that it was always too dense and you ended up always being limited by weight downs and or silicone...me, i change my tank around enough to not want anything too fixed to use silicone in my tanks.
Plus, quickcrete has a chem in it that basically makes it harded fast (thus quick) but that chem also makes it weak (compared to concretes). Them chem in concrete that makes it strong (1000K 5000K ect) is toxic. So this slow cure will leach a chem in the tank that will kill the fish /plants...Quickcrete will not and will cure with in 72 hours (plus is non-toxic, as it is sold in some stores as a craft/hobby crete...kids, projects type stuff)...
long store short, quickcrete is safer then 75% of the aquarium decors sold at LFS (which 60% of are not even intended for aquarium use)
Greatstuff: this is toxic during cure times, so follow directions and plus 3days, but here is a cool factoid...greatstuff is water proof and in activated by air inorder to harden.
This is important, that you allow the greatstuff 2-3days to fully cure, dont pile a large amount at once (little at a time if your doing a big piece, so that the air is able to cure the inner areas) and Tempture will, re activate great stuff to swell....
how to control this?
spray the greatstuff, let it set for two days, cut out areas sections or carve (knife and sand paper) then hit with the heat gun or hairdryier, this will make sure that any chem agents left...are removed...re sheape if needed and ...cover with your crete...over all 7-10days depending on your design will be needed.
want to really do something cool...mix a lil arograte,crush coral or sand into the mix...paint?!, yes you can...quick crete dyes are non toxic too...or you can use a non toxic acrylic paint of your choice. this will now give you a rock that you really want and see in your mind verse what you can find ate the LFS or in woods,creeks ect....
i have made, faux rock,coral and driftwood all this way, single pieces and full backgrounds, personally and professionally...never had a death related to this method.
how did i learn of this?!
Aquarium company many moons ago who made faux biotopes for aquariums (such as camden aquarium)...
on large designs, 2pt foam can be used but the draw out time is much longer
ill have to post up some pics of what i have done before to give you some ideals.
goodnews on the tank...
im back down to 76* although the HVAC is still not done...I barrowed a window unit and set up in the living room...yup....cooling the tank down...
phew, what we do for these fish...just hope this SC 80+* sun stays at bay til the repairs are done