Jay,

Neutral is best, generally, but it's really dependent on which species and where they come from.

5.8

It has been argued by Elton Roberts and Malcolm Burleigh that even 'alkaline' adapted cacti growing in limestone soils actually experience acid conditions during the wet season because rain water is always acidic, and limestone dissolves slowly enough that the calcium carbonate does not really raise the pH at the roots above neutral pH 7. During dry times, roots are not exposed to alkaline or acid conditions.

For more on this read the discussion here at GW: Tap water pH, and the original paper by Roberts and Burleigh that inspired that discussion, Cactus and Alkalinity.

Brad

6.5 to 6.7 they will be happy campers, rain water will solve the problem,