Effect of RODI on alkalinity

[kartrsu]

Hi @Randy Holmes-Farley. My LFS advises me to add alkalinity to my top off since RODI has an alk of zero which they say dilutes the alk in my tank, but I think that's incorrect. If I recall my chemistry correctly, when water evaporates, alkalinity should rise until you replace the evaporated water, which brings your alkalinity back down to original levels, assuming no coral consumption. So adding RODI should not have a dilutive effect on alkalinity. I did wonder though, since ions can leave through gas exchange, does CO2 movement in and out of the water affect alkalinity a whole bunch? Thanks!

 

[Opus]

Chemistry aside, I think what they really meant was your tank should be "consuming" alkalinity and your top-off is a good place to add it back. My top-off is around 5 gallons a week. I put sodium bicarbonate in my RO to keep my alkalinity stable since it is adding water every hour or so.

 

[Randy Holmes-Farley Verified Community Expert]

Hi @Randy Holmes-Farley. My LFS advises me to add alkalinity to my top off since RODI has an alk of zero which they say dilutes the alk in my tank, but I think that's incorrect. If I recall my chemistry correctly, when water evaporates, alkalinity should rise until you replace the evaporated water, which brings your alkalinity back down to original levels, assuming no coral consumption. So adding RODI should not have a dilutive effect on alkalinity. I did wonder though, since ions can leave through gas exchange, does CO2 movement in and out of the water affect alkalinity a whole bunch? Thanks!

You are right, they are wrong.

CO2 cannot have any impact on alkalinity, whether added or removed.