calcium raeactor or two part dosing?

I would think with that much water you would be better off to keep it like it is.

 

If you already have a CA reactor and Kalk reactor why would you even consider going 2 part? Especially considering once setup properly the CA reactor is basically automatic.
With that much water volume you are going to have to dose huge doses so it's going to get pretty pricey.

Are you having problems with CA reactor not keeping up or something? If that's the case you could always dose to make up for it.

 

Don't worry about your pH. If I told you what mine was at, you would laugh and not do anything about yours.

I agree with Jeremy and stick with what you have; providing it works. If you need to adjust anything, just correct it with 2-part.

 

PH is completely overrated.  As long as you are in range don't worry about it.  If your Alk is continuously dropping I would say either your CA reactor isn't adjusted properly or isn't adequate for the size of your system.
 

 

This might help you tweak it a bit.
http://reef.diesyst.com/reactor/reactor.html