Alkalinity, Calcium, Magnesium all high. Concern?

Hello:
I'm new to reefing and have a question. Here is the setup:
- Innovative Marine Fusion Nano 20 gal. tank with skimmer, carbon reactor.
- Lots of LPS corals, some softies, 9 small fish
- Tank is a couple of months old
- I do a 20% water change every week with LFS water.
- All evaporated water is replaced with RO water.

Readings:
- Temp: 77.2º
- PH: 8.3
- NO3: 0
- Ammonia: 0
- Alkalinity: 12.3 to 13.4 depending on which tester I use
- Magnesium: 1400
- Calcium: 470 to 480

This is what I get almost all the time. These numbers are higher than what I've read they should be.

Question: Is this a cause for concern? Is there a way to reduce these numbers without more water changes (that make no difference)? Anything I can add to the tank to reduce them?

Much appreciated!