Have you tested it as acidic from the tap? Check your tap water first, as I have seen people have pH issues from using containers to fill their aquarium that have been washed in a dishwasher, amongst other things. If there is any possible outside cause, try to find it because constant tweaking of pH is difficult to get consistent and hard on the fish. Also, since the fish that are dying are generally very hardy, and pH issues often affect fish much faster, I would wonder what your other parameters look like and what your cycling method is. If pH is definitely the issue and it's the same in the aquarium as from the tap, using crushed coral as a substrate can often work fairly well for raising alkalinity.