Most tap water contains minerals, and those aren't good for the system, even with the green anti-freeze. You should always mix with distilled water when possible. That is the recommendation.

Having said that. I use my garden hose to flush the system, then add anti-freeze, and top off with distilled water if I have it laying around, more times then not I just finish it off with the garden hose tap water.

 

I finally did find Prestone dex-cool 50/50 quick fill mix. This should be ok for the top off right? If not, I'll buy the regular dex-cool and use the distilled water for mix.