Buffers and RO/DI units
 

Do I still need to "buffer" the fresh water top-up water once it has passed through my new RO/DI unit before I add it to my evaporation-depleted tank?

If you measure the water right out of the unit you'll find it to be in acidic.

When I was adding a gallon at a time from jugs, I would aerate first but since running a float valve autotop-off I don't worry about it.

thanks for the info. I'll aerate then rather than use chemicals

If your water is acidic coming out of your RO/DI something is wrong. It should be pure water, aka neutral, aka 7.0.

You don't have to add buffer to the water if you are using it for top off as the majority of the calcium and such dissolved in it would be left behind when it evaporates. Many people do add kalk or other things to top off water but that would be considered dosing / supplementation.