A 1.5 pH is extremely acidic, close to the acidity of hydrochloric acid. That doesn't make sense for your tank to be that low, as CO2 saturation in water will level out in the high 5s for a pH and that's with a ton of CO2 in the tank. Is there anything you have been putting in the water? Once again, how old and what type of test kit are you using? With a pH that low, ammonia isn't even toxic, as it's ammonium.