If it had chlorine the ammonia would've spiked and I have 0 pmm of ammonia.
An ammonia test will not detect chlorine. You can easily have high chlorine and no ammonia. While your ammonia levels are fine you haven't listed any nitrite levels which is also very dangerous to fish. Your nitrate and PH are fine.
If you have a lot of organic material in the soil microbial activity will release CO2, CH4, HS and H2SO4. If you had a lot of CO2 your PH would be lower. Many people here also inject CO2 into their tanks for faster plant growth. They often have a PH between 6 and 6.5 with no reports of red gills. HS and H2SO4 are however very toxic to animals. Unfortunately I don't know how to test for these.