I have a Hydro and, as far as stinging, he will let other coral grow within roughly 5cm of him before he stings, but when he does, wow! He will kill over 15 cm (at lest 2') away from him.
As for polyp extention - this is actually a great topic. I have great growth and color on my colonies, but in many cases very little PE (compared to when I bought them). I know flow has some bearing, as does light, and tank conditions, but for the life of me, I can't pin it down to one thing. I do notice better PE after a water change though. Maybe old water is causing issues?
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