My rock is real hard, has been in water for 6 days now. Daily water changes. I will go 5 weeks and then start testing the PH
. Some good ways to build caves that I used are to make your rock in layers. I put sand in the middle of the layers and build up pockets. When I layer the concrete over these "pockets" the mixture sets up and hardens around. I then wash out the sand using a hose and it leaves pretty neat caves/holes in the rock.