I made all teh rock in my tank using Oyster Shell and #10 cement. I left them out side in a kidy pool for a few month. I think I made them in Feb. and placed them in the tank in June. Make sure to change the water at less weekly. I also gave the rocks a 50/50 vinegar and water mix twice during teh process. Man does it fizz the first time. after the fresh water I just placed it in a bare tank, with sand and a few small LR pieces. I had a high PH for three days or so, then it was normal. This has been two years now and the rock looks good in the 33g and covered with coraline.
here are a few pics when I changed out the playsand for crush coral over one year ago.
