Ok, in that case, that sounds very odd. If the co2 tanks are at or above 5.5, that sounds normal if you have low KH. But in a non-co2 tank, 5.5 is pretty acidic. GH itself has no effect on pH, it's only the KH that buffers the pH. I add magnesium sulfate and calcium chloride to my tanks, it only increases GH and never affects pH. I'm not sure what else might be in the GH booster that would cause a pH drop.