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How important is Ph? Mine is running 8.4 and I think that is too high. At present I am using softened tap water. I can get a RO unit if necessary, but I am concerned about the lack of minerals.

Am I unnecessarily worried again?

8.4 ought to be fine. What should concern you more is whether the pH is stable. I have never kept any of your fish, but most fish need stable, clean water conditions more than they need specific pH or hardness. If you really want to change your pH, you should do so with RO, as you suggested, or with rainwater. Catching rainwater is simple and free, so thats the route I would suggest. Its working really well for me. (My pH out of the tap runs something like 9.8, which is incredible). Your water may decrease in pH as your bio-cycle goes along, as this produces acid in your tank. The bio-processes never helped mine, so the only choice I had was rainwater. It works well enough, though.
All in all, though, IMO your fish should be fine in 8.4 water if its stable.