Why are my zoanthids wilting?

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Some brown zoanthids came in on my live rock a few months ago.

They did well at first and propagated nicely.

For the past week they look sickly, the tentacles are thin and wilted, the body of the zoanthids look flaccid.

My parameters all test out OK: temp 80, pH 8.2 to 8.3, zero ammonia/nitrites/nitrates. The rest of the tank inhabitants are healthy (assorted soft and hard corals, crabs, 2 clownfish).

I have narrowed down the possibilities:

1. Too much light -- they are high up on the rock under fairly intense T5 lighting.

2. Lack of nutrients -- the tanks is fed sparingly so far due to low fish load.

3. Depletion of trace elements -- no water has been done yet, I had really ought to do one.

4. Allelopathy -- I don't know is this concept is for real or not.

Any advice on this would be helpful, thanks.