Most of my rock is DIY. Made a lot of hollow rocks for fish to swim thru. Used Caribsea sand, crushed coral, and concrete. Filled the cavity with dry sand as I made them. Poked them with a screwdriver to make a bunch of holes in them. Once the rock was dry, just poured out the sand. I made a few rocks every day till I ran out of sand and crushed coral. Made about 100# of it. Cured it in a trash can. Added some vinegar to help get the ph down. If I did it again I would add a little extra concrete.