That’s a neat idea – the only snag is that most concrete really does leach calcium and concrete crushes coral. You could look into low‑leaching binders, like adding polymer admixtures or using geopolymers that are less corrosive. Or go totally different and build a modular reef‑friendly frame with fiber‑reinforced composites that still lets water flow. I’d love to run some tests in a tank first – no one wants a little reef disaster just because we got bored with bureaucracy and built a hard‑shell box.