I have a saltwater reef tank and i am looking at my water source. I have a 3 stage RO filter (sediment-pre/carbon-RO) My tap water pH shows 7.5, RO output is 5.0. I thought my filters had expired so i replaced all 3 and flushed. I still show a pH of 5.0 output. Harrisburg water is setchey at best. Are they buffering with something the RO removes?
Hi, yes it seems counter intuitive that deionized water has a pH lower than 7 (or pure water), I too have questioned this even in lab grade deionized water. You are correct though that reverse osmosis removes all the buffering capacity of the water, largely carbonates and other ions that buffer and affect pH. Since RO or deionized water comes out ion free it readily dissolves gases like carbon dioxide. When carbon dioxide dissolves in water it forms carbonic acid and will quickly lower deionized water pH as the water has no buffering capacity. When you use the water for reef tanks the salt mixes used will quickly bring back the buffering capacity of the water, but care should be taken that the pH stabilizes before it used for the tank.
I hope this helps.