Hi Bychineva, not sure if this has been properly explained, the brown colour is caused by the symbiotic zooxanthellae, tiny little photosynthetic organisms that live within the coral. In nature the coral has these living within its tissue, because they perform photosynthesis and the coral benefits from the sugars released.
In the wild, coral reefs occur where the water is very pure, it is very hard in our tanks to keep the water as clean as it is in a wild reef. So often in a tank, there will be more crud in the water than corals are adapted to, and this supplies nutrients to the zooxanthellae within the coral, causing a population explosion of them. This makes the coral brown, as the zooxanthellae are brown. In turn, extremely high levels of zooxanthellae can be harmful to the coral.
The coral being brown is not nessecarily a cause for concern as many corals are brown as their natural colour. However, a coral being more brown than it should be is generally a sign the water is less pure than desireable, although there can be other causes also.
As we cannot test for all the forms of nutrients that can overload our tank, we test for one in particular, phosphate, which gives us a rough idea how the others are going also. IE, if phosphate is high, it's odds on that other nutrients will be too high also. Because even tiny amounts of phosphate can be too much, it is likely that if your test kit can detect any at all, there will in fact be too much. Another way to tell is by algae growth. If you find you have to clean the front glass quite often, this will indicate nutrients are too high. Hobby test kits are at the limit of their range trying to accurately detect the low levels of phosphate we are interested in, the glass cleaning test can in fact be more useful to us, and we can gauge if things are getting better or worse, by wether we are having to clean the glass more or less often, than we used to.
As to your dosing, the iodine and strontium are not ones that are normally associated with nutrient overload causing browning so are probably not the problem. However, there is often little/no need to dose these. Bio trace, I don't know what it is but many of these type of multi trace element type formulas end up putting too much of something in the tank. Very often, when people in exactly the situation you are in, stop dosing these type of products, they notice an improvement in livestock health.
But the main source of excess nutrients will be the fish food. Nearly all newbies feed to much, it is so tempting to keep those little fishies fat. Try cutting back the fishfood and as long as the fish don't get skinny, hold it at the lower level.
The biggest thing of all, is that export of crud from your tank, has to equal import. You need to look at your skimming, sand vacuuming, & whatever else you do, to ensure you have processes in place that will take out as much as you are putting in.