Tetras are usually soft water fish and may not do as well if the pH was too high. However, guppies actually do better in harder water with a higher pH and bettas are quite adaptable and should not have any real issue with it either.
I will not darken the door of any store that focuses exclusively on pH as some sort of 'problem'. Ammonia and nitrite are what can kill fish, rarely does pH come into it, unless you are dealing with sensitive wild-caught fish.
Trying to chemically alter your pH generally does more harm than good. If you have water with a high carbonate hardness (KH) it will have a greater buffering capacity and be more resistant to fluctuations in pH. This will be very difficult to undo using chemicals and more importantly, difficult to maintain stability in both hardness and pH.
Right now I would be more focussed on testing for ammonia and nitrite if you are getting high readings for ammonia, as both can be fatal to fish at very low levels.