I test my water with the Master Test Kit (expires 8/2018) at least twice a day and have for the last 7 weeks. Trusted it completely. Fish started acting strange last Friday. Checked and rechecked my water. All was normal but changed water anyway. Saturday AM 5 dead fish including my Rainbow Shark who I REALLY liked. Changed water, made a 45 mile trip to town to get test strips to see if they would show different than my Master Kit. Sure enough the Master Kit is showing 0 Nitrite (beautiful blue color) but the strip shows 5.0 Nitrites. Absolutely makes me sick to think I trusted something so completely. Be WARNED that kit is not the end all that people make it out to be. Have no idea what caused the spike but sure wish I had known it was happening....