Strange pH question

[Solarreef]

My tanks pH has been always depressed: around 8 or so. My animals all seem to be fine however. With my recently bough ph meter I have found something interesting. I mixed some saltwater on saturday. And the ph was fine at first: around 8.3 but then several hours later it was down to 8.15...then 8.05 and then I checked again today and it was 7.9. Do you think it could be from a faulty deionizer/reverse osmosis unit that i have. I just recently realized this because of the ph meter. Note I did not do anything to the water besides heat it and use a powerhead.
thanks,
nils

 

[Solarreef]

I forgot to add a few things. The salt was instant ocean and it was the correct salinty: 1.023. What makes it even stranger to me is that the water straight from the reverse osmosis/ deionizer is around a pH of 9.
nils

 

[randy holmes-farley]

The pH in your tank is not a concern, and the pH of the salt mix is also not usual.

What you are observing in the salt mix is that as it equilibrates with Co2 in the air, the pH drops. Many salt mixes initially mix very high (some 9+) and then the pH drops as they suck in CO2. Here's an article that shows that for many different salt mixes:

http://www.animalnetwork.com/fish2/aqfm ... efault.asp

The pH of pure water is almost meaningless, both becasue it can jump around wildly becasue there is no buffering present, and becasue pH electrodes don't function well in very low ionic strength. So there is nothing of concern in the RO/DI water. FWIW, if I stick my pH probe in it after being in my tank, it reads about pH 10, even though it is pure water.


Here's an article on pH issues in reef tanks:

http://www.advancedaquarist.com/issues/ ... 2/chem.htm