So while anything in theory could built up with insufficient water changes, if you trust your maintenance and tests then moving forward start testing your foods like Nori which I think is rich in iodine.
I would think that if the test is truly accurate at measuring trace amounts of things, it could show elements from things decomposing that you put in your tank. I doubt the fish is going to absorb it all so possibly a lot of iodine is pass on through waste.
I somewhat agree with the one poster about things building up over time. You have to do a large enough water change to over come the single pollutant you are trying to lower in that same time frame, its mathematical.
You could in theory reduce the iodine input or increase iodine export, only you will be able to determine what is practical and fun.
If you truly want to maintain "factory out of the box" water parameters, water changes would have to be well over 50% to actually over come these auxiliary inputs right from the start. Again, its math not theory, just depends how you want to go about it, how much dosing you want to do, or your laziness to risk factor.
If you add 2ppm of iodine per month but only change out 25% of your water per month, by the end of next month you will have 3.5ppm, 4.6ppm by the third month, 5.45 in the 4th month and so on.
Lately I've been pleased with the 30% every 2 weeks as a compromise. Even then I realize that it doesn't hurt to get a bit of a "reset" in there with a >50% water change, just not nearly as often because 30% every 2 weeks does a reasonable job at exporting things that alter the parameters.
I think 30% every two weeks is closer to that magic tipping point of actual maintenance vs controlling a slow motion crash via slowly climbing elements and pollutants. Dosing to extend water life has to have compromises in dealing with buildups.
If you compare this water against factory water you will get your answer. I think it's fair to say that since you have nitrate and phosphates registering it is evidence that the conditions are there for a buildup of other elements regardless of the source.
I appreciate you sharing the test results. If its a concern I would change more water volume more often and not worry about it, if it's a OCD curiosity test some fish poop or nori infused water. This would require sending in a fresh saltwater sample too though.
