Common symptoms of low pH is nervous movement in fish. Fish may dart across the tank. Death of new fish which are introduced into the tank can also indicate low pH.
Common reasons for low pH are overstocking of the Aquarium and inadequate tank maintenance.
The Baking Soda should be added in small amounts. 1 teaspoon for five gallons. Remove all the fish to a separate tank before you begin the dosage.
The pH should be changed very incrementally. Drastic and sudden changes can kill the fish.