Ph down chemicals?

For the last few weeks, my new tank has been very steady at the following

Before lights are on:   Ph: 7.0, KH: 6-7, CO2: 18-21 milligrams/litre
After lights are on (for 4-6 hours) Ph 7.2-7.5, KH 6-7, CO2: 10-14

From what I have learned, read and seen, a little lower Ph and KH would be
more ideal. Like around a 6.8 Ph and a KH of 2-4; I am using DYI CO2, so
cannot inject anymore CO2 than what is being produced by the organic rig;
and so was wondering about using a Ph/KH down acidic product (such as
Tetra) to slowly (over at least a week or two) lower these levels, in the
tank water, and to lower my tap water for water changes as well (tap is
7.5).... The main chemicals I believe are hydrochloric acid and sulfuric
acid in these products...?

Todd March
tmarch at primenet_com