Answer Baking Soda. But what make you think a ph of 7.8 is causing fish death? What time of day did you test ph?
If you use the Baking soda or washing soda approch you need to be very careful, you can send you ph skyrocketing causing all kinds of problems that could make you do a complete water change. If you try it use caution and start out with a min. amount and test, test, test your water.
Baking Soda
To raise 50 gallons of tank water by 1 meq/L will require about 16 grams of baking soda (sodium bicarbonate; sodium hydrogencarbonate). Since a level teaspoon of baking soda weighs just under 6 grams, then 1 teaspoon will raise the alkalinity in that 50 gallons by ~0.4 meq/L (~1 dKH).
Washing Soda
To raise 50 gallons of tank water by 1 meq/L will require 10 grams of washing soda (sodium carbonate). Since a level teaspoon of washing soda weighs just over 6 grams, then 1 teaspoon will raise the alkalinity in that 50 gallons by ~0.6 meq/L (~1.7 dKH).
Thomas
