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 except the pH was suddenly 8.6! I use an API liquid test kit and tested it twice to make sure I was getting the correct reading. None of my other fish seem to be distressed, but i'm sure they are with such a drastic pH change. I have 3 cories, 3 (now 2) zebra danios, a mystery snail, and a single female betta in this tank. (before anyone says anything, I know the zebras shouldn't really be kept in a 10 gallon tank and that i should have more of them. I was going to get 2 more in a few days, was waiting to make sure the bio load was under control, I just got the danios a week ago)
How do I get the pH back to normal? I'll do a big water change, but what would cause this?
10 gallon tank
nitrate: between 5 and 10
pH: 8.6 (normally 7.5)
Thanks for the help!