My local reef shop said to use at least one pound of rock per gallon for filtration and such. Since its a 36 gallon tank, i purchased 40 pounds of caribisea life rock. I laid out the card board pattern on a table. Marked off 2 inches around the whole card board box to allow for sand area and ability to clean inside the glass. I'm telling you that 20-25 pounds of rock about a 12-14 inches high is all i will be able to put in my tank. Anymore and i will not have any room for any kind of swimming fish to have room. Whats going to happen to my water balance if i cant put but half of what is recommended for a 36 gallon tank? I will ultimately put in a protein skimmer in my sump tank so that will limit adding much if any rock in there? This question is open to all and any that can give me some advice. I have enough rock to fill the cube and make the aquarium look like a Prison Facility. Thanks