If the fish didn't die in the first few days (really even the first few hours), it isn't going to be pH related. The long-term effects of living in non-native pH waters are pretty minimal. Primarily, the fish may never receive a spawning trigger. But, not spawning is not a cause of death.
Fish are exceptionally adaptable to pH changes. The pH of natural bodies of water (lakes, rivers, streams) change during the day and sometimes very rapidly. At the temperature of the bodies of water change, so does the pH. Aquatic plant activity -- which is dependent on whether the sun is obscured by clouds or not -- changes the pH, too. Temperature and aquatic plants have been shown to change the pH over 1 pH unit throughout the day, and the fish in the body of water are perfectly fine. Secondly, runoff from rains is almost never the same pH as the body of water, yet there aren't massive fish kills after every heavy rain. Finally, in the large bodies of water like the African Rift Lakes, there are layers and/or regions of water that are different pHs, yet fish can move about through the different regions just fine. pH gets blamed for a lot of issues that are rarely actually pH.
First and foremost, sometimes fish just die. Where they adults when you bought them, or juveniles? If they were adults it is usually pretty hard to know how old they were.
Secondly, you probably should describe much more detail about your tank. How big is it? What is/was the stocking list? What kind of maintenance schedule do you keep? Specifically how large and how frequent of water changes do you do? What are the current readings of all the water parameters (ammonia, nitrite, nitrate, pH, GH, KH)? What chemicals do you add to your tank? What are the most recent addittions -- plants, animals, decorations, rocks. If you tell us all this, the forum will be able to diagnose your problem more accurately.
p.s. FYI, it is properly written pH. little p, capital H.
