If you have chlorine in your tank, I would say the sediment and/or carbon set of the RODI unit are exhausted.
I have no idea where arsenic would be coming from. But that should be filtered out of your rodi as well.
Some salt mixes have higher or lower levels of various components like Iodine, strontium, etc. I would see if you can find any threads on R2R discussing that with reef crystals. Also, anything that molts will use iodine. I don’t know what your shrimp & pod count is, but that may be why? Iodine is not easy to test, but you can. And then dose accordingly. Go slow! I’ve never tested strontium.
I would never dose something I can’t test for. Except for Aminos.