Different fish require different levels of PH, therefore it is wise to keep fish of similiar PH levels only in a tank. The more plant life you have in a tank, the more basic the water will be. The more fish, the more acidic the water will be. Most fish like a PH that stays basically neutral. High acidity of alkanlinity can cause problems, like damage to skin, gills and eyes.
Fish become stressed, and lifespan can be severly shortened.
Death rates will rise. Aquarium Pharmaceuticals makes a product called Proper PH which you can toss in in your tank to make PH a specific level.