ph spike

Ok folks so I ran into a pretty nasty issue today, I was doing a video for my vlog and I was testing Ph and noticed it was pretty high. So I got out my high range ph test and I'm above 8.0 from what it looks like. The only thing I added besides dechlorinated tap water was some black gravel from...

My tank just had a sudden pH spike. It is normally around 7.5. I tested it yesterday morning and it was 7.6. I woke up this morning and one of my zebra danios was dead. It hadn't been showing any signs of illness or disease, so I immediately tested my water. All the conditions were normal...