TDS is a measure of conductivity of the water.....anything in the water will increase the conductivity.
Copper can come from
Any old equipment/rock/tank that had copper in it, it will leach for a long long time
Did you buy a new fish recently, and get any of the shipping water in your system?
What about plumbing....do you have copper piping anywhere near your tank.
I do not believe that carbon will get metals, just keep up on your water changes. but Al and Li are in the Reef Crystals. Also note that Magnesium comes from Part 3. You could stay on KalK and just get some Mag to does.