as long as the tap water is sierra tap water (melted snow) it comes through at about 7.0pH less than hundred ppm of total disolved solids. i do not us RO at that point. it is when the batch plant mixes in well water that can be over 8pH and tds of 400 or better that i use RO. but all of the water runs through a GAC tank prior to being added to greenhouse plants, koi pond or fish tanks.
the purpose of DI resins is to exhchange out unwanted elements of the water. i do not add chemicals to the water and prefer not to use any buffering solutions such as pH up or pH down that just adds one more thing for the fish and plants to deal with. some prefer to use DI resins over RO it is more convienent when recharged than dealing with a RO holding tank. to me it requires more work because it has a ppm limitation between recharging. it is not uncommon to mix several different methods of dionization to achieve the desired results including multistage distilation or a slow natural form of ion exchange by using acid peat, most of the peat available in stores lacks the low pH of 3.5-4 and can be partially neutralized.