Most people are very hesitant to put concrete in their aquarium, and justifiably so! Concrete should be cured for 6 weeks before use in any aquarium, but after that, there are no adverse effects. The following link is an article which will give you the concrete recipe and instructions on how to make these caves and arches. Be sure you you have an hour to read and ponder this one...it's THOROUGH): Making Your Own Live Rock.
