I have done this and IMO if you have the time to let the concrete cure and leach its high PH in RO water, its much better than PVC structures or zip ties. It ends up looking the most natural. Once the concrete grows over with coraline and whatnot, the rocks look like one big piece.
I think I used portland cement. Whatever it was, nothing fancy, cheap and never had problems. I had that tank up 3 years.
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