When we first set the tank up, We were using nitrifying bacteria in a bottle. It must have been old or inert because when we added fish we got the ammonia spike. It took two weeks to get that situation under control but we did and started getting good readings on the tank. Four days ago the pH was 7.4 and then yesterday it had dropped off the charts and was less than a 6.0. We added pH up and then today the fish had white fuzzy stuff all over them and within 12 hours they were all dead. Have you ever seen anything like that before? I shared a picture earlier in the chatalso a quick change in PH can greatly affect fish negatively
And as itiwhetu said, guppy prefer an alkaline PH
Did you properly cycle your tank when you set it up? An ammonia spike would indicate it may not be properly cycled.