Help My Ph Is Going Crazy

[fishymaster66]

well i ph was 6.5 which is what i wanted it to be, but when i did a water change, with aged water nitrite 0 nitrate 0 ammonia 0(i added something) and Ph 7 BUT when i added it in and a day went by, the ph suddenly dropped to 5 HELPP HELP an angelfish and a neon tetra died Help HELP

 

[WILDER]

Have you been using a ph adjuster.
What is your tap ph.

 

[fishymaster66]

it's 5.5 but i added ph 7 so doesnt that make it betteR?

 

[newtotropical]

I doubt your nitrates are 0
What test kit do you use?