Molt help, remove chem filtrants?

[Aurortpa]

I noticed my purple coral banded shrimp had a not so good molt. He lost one arm and his “antenna” look all scraggly and short.  

 

I know she can survive, furthermore it will all regenerate with the next molt but wondering if there maybe a specific cause or if this a random anomaly that just happens sometimes?  

 

Alk 10.5, Cal 455, Mag 1390, nitrate <10, & spg 1.025. I do weekly water changes of 10% with red sea, run a small amount of carbon, and small cutting of Polyfilter.  Skim aggressively. My only guess is my water is stripped of some kind of element she needed, is it just possibly iodide? 

 

If that’s the case, I am wondering if my filtrants strip elements essential for maybe corals too then? I used to run a 10g with only liverock rubble in the back chambers—no filters. Any feedback is appreciated!

 

 

[A Little Blue]

Dose iodine.