Welcome guys!
Here's some good reading for you;
Chemistry and the Aquarium
And
http://www.reef-keeping.com/issues/2.../rhf/index.php
Take the "-" from between reef-keeping. I couldn't get the link to work properly.
The only thing I'd be worried about is bringing the PH sightly up and the Alk down in your 20g. Your 75 is ok, I would raise the PH a bit. This is all assuming your Alk is being tested in dKH not meq/L.
High calcium concentrations will cause no harm to your tank. As a matter of fact, I try extremely hard to get, and keep, mine in the 450ppm range.
The last thing I would do is add limewater (kalkwasser) to your tank.
1. You Ca is high enough.
2. Limewater has a natural PH of 12.5
3. Limewater has a natural Alk of 40 meq/l
4. Limewater has a natural calcium content of 800 ppm
Limewater is used, and best used, once your parameters are stable.
I would just do water changes to try to correct your Alk and PH issue. If you try to raise your PH, you will in turn raise your Alk.
What brand salt are you using ?
Some are very high in Alk, and can make things worse.
I would also suggest getting your own test kits and monitor your tank. You can also get free software to keep track of your parameters. I use Aqualog, and have found it very useful to track my progress,
AquaLog