I built a " wall " around the back and sides of my tank using aragrocrete and built some rocks too
My formula was 1 part argonite / 3 parts crushed oyster shell / 1 part portland cement & about ( give or take ) 1 part water
For the " wall " I poured this mixture on a piece of eggcrate with plastic vapour barrier under the eggcrate on my basement floor ... then cured it in a kids wading pool ... the curing took forever
For the rocks I did much the same as you as far as the forming them and so on ... the testing of the PH and frequent water changes is key IMO
Cheers