After the initial week of topping up the chemical sink, a 1000 mls of Trace Elements will last a medium stocked 1000 Liter system, around 2.5 months.

Yes, but not if you follow the dosing instruction. Three monthly ICP test are recommended.

If you are using Randy’s 2-part mix or a commercial blend, it is important that the correct trace element solution is added to the correct chemical solution.
At rate of 10mls/Liter: -
Solution 1 is added to the Magnesium solution.

Solution 2 is added to the Calcium solution.

Solution 3 is added to the Alkalinity solution.

It is a liquid version of GFO. The phosphates bind to the iron oxide and get removed by the skimmer or by mechanical filter.

Some Tangs can be sensitive to elevated iron but will adapt with smaller doses over time.

Phosphates can come in from foods and leaching out of rocks. Soak 1 gram of food in 1 Liter of RO water and test it for phosphates.