24/7 is always the best choice when mixing SPS w/ leathers, I would recommend it regardless. Though Sinularia and Sarcophytons are known to be the highest to exude terpines into the water column and wage allopathy on any would be encroachers.
I hope I never gave the wrong impression that carbon should be used for a only a couple days, on the last thread that got kinda ugly and the topic easily misunderstood amongst all the poo poo flinging.
I think it takes the carbon a long time before it starts to effect trace elements, unless your keeping your water for more than a month or two, through your testing you may find that supplementing might not be necessary, especially with your weekly waterchanges. Having so many leathers and xenia and a few clams might be where your iodine is getting sucked up if you already notice it's depletion a little higher than you may have before.
At any rate your doin better with them than I ever did mixing them. Tried breifly on my 30g a while back and they within days started to fade poor polyp extension etc, I had a 175MH on them so I shouldve been able to at least keep healthy brown sps, nope. I think it was the leathers, and the occasionaly film they sluff off definately doesnt help either.
There some tanks and keepers that can pull it off though, and everything I've heard more often than not is contributed to constant carbon usage. Look forward to hearing your findings
-Justin