Clorine problem

Well, since iodine is a halogen, like chlorine, it seems possible that you're overdosing and the dechlorinator is reversing the effect. Basically, there must be an oxidizer in there somewhere. If you were using ozone, that would be a possibility as well.

Try stopping the iodine additions. According to Borneman's book, some species of coral are iritated by iodine supplementation. How do you know that you need it?

I take it you didn't like my first answer. Maybe I need a little more info, because massive iodine overdose was my best guess.

Anyway, it has to be something that reacts with thiosulfate. That would be halogens (e.g., chlorine, iodine, bromine), and probably a bunch of other stuff. I'm too tired right now to hunt around, maybe someone else will have a clue.

The other possibility is that your Ca reactor is pumping out something evil. That would presumably show up in the alkalinity and Ca levels, though.

So maybe it would help all of us if you could tell a bit more of the story. How long have you been fighting this, how many fish and of what kind have you lost, what are your current water parameters?
How did you figure out that you need daily dechlorinator in the first place?