River rock. Its rounded and cheap (Pike stone center). The only drawback is its weight. An avalanche in a 55 doesn't go far, but it might cause trouble in a 220. I like those "plastic rocks" and Lok-rocks (cheesy looking, but the fry hide underneath). But in any quantity, they get prohibitively expensive.
PVC pipe. You can fasten it together into pyramids with cable ties or hide it under a rock pile.
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