What is the best pH range for a healthy aquarium?

That just depends on the kind of fish. Some fish like tetras need acidic water with a low pH meaning the water is more acidic. Some fish need basic water meaning it has a higher pH. There is no hard an fast rule, you have to research the individual species to find out what pH range is best suited to that species. That number is usually based on the type of water conditions found where the species grows in the wild.