Help understanding PH in 90gal

[Philly33]

The replacement water 45gal is ph7.2 kh5
Once the water change is complete ph is 6.9

Is there residual CO2 that gives me the 6.9 reading? I'm thinking I know the answer but wanted a second opinion.

Thanks in advance

 

[Mebbid]

Water commonly changes pH as it sits and degasses. Just take a bucket of water, test the pH, and then allow it to sit overnight before testing again.

pH can also change as part of the day / night cycle as well.

 

[Old Scales]

What you are seeing is the fresh tap water is charged under pressure and chemicals added that sterilize the water and temporarily keep the pH at neutral or slightly above. This helps prevent long term corrosion problems in the water system's distribution lines. After the water gasses off these treatments, the Ph lowers to its true level. If you have 6.9 natural pH you are lucky unless you're keeping African Cichlids. OS.

 

[Philly33]

Let me refresh. My ph controller is set to 6.6. I did a 50% wc. After the wc my ph was 6.8. I want to know if there's residual co2 in the other 50%? Prior to this wc my ph was 7.1 after a wc.

I don't use tap. I have RO. The water is made on a Saturday. Sunday it's heated to 77 and additives are added with a nano fan to circulate.

 

[Mebbid]

That's hard to tell... are you using straight R/O water or remineralizing it?

 

[Old Scales]

OK The replacement water has less CO2 than the charged CO2 in the tank. Co2 in water lowers pH. You can easily have a 0.3 variance in different WC's.

 

[Philly33]

Thanks. That's what is thought.