As long as you have pods, your mandarin will be happy. I'm BB in my 250, and I have a ton of brittle stars, and a TON of pods in there (posted a video recently). The brittle stars either stay in the rocks and wave their arms in the current, or lay underneath the rocks. They don't seem concerned in the least that there is no sand. I will be adding a mandarin (or pair) in the future, but don't have any right now.