I agree: Dichlor is a good option as it will add CYA as it chlorinates. It WILL tend to drive your pH down so be ready with Borax or lots of aeration to maintain.
I can't give you all the chemistry as to why Dichlor which is pH neutral, drives pH down anyway (but not nearly as fast as Tri-Chlor), but it has to do with chlorine in the water is acidic and dichlor doesn't counteract it.

Carl