Low PH when mixing salt with RO water?

[Jeff_S]

When mixing up fresh saltwater using my RO unit the PH is usually around 7.7 to 7.9. Is this typical? I am using Coralife salt.

 

[Jeff_S]

After testing the RO water prior to mixing any salt I found it to have a PH of about 6.

The tap water before the RO is about 7.4

Is it normal for an RO unit to lower PH this much?

 

[jdeets]

Jeff--it is normal for RO/DI to lower pH significantly. My tap water is over 9 and my RO/DI is about 5.5.

Test the alk and Ca level of the freshly-mixed water and see if you might need to raise the alk any. (Ca should be fine, but wouldn't hurt to check anyway.)

BTW, I've seen this same problem on other threads. The best way to raise the pH (assuming the alk of the new water is not too low) is to aerate/circulate the new water for several days before doing a water change.