Tap water via water softner and RO water mix to raise kh

[Philly33]

90 gallon heavily planted Co2 injection w/ ph controller EI dosing 4 x 54w t5ho (2 midday 2 aquaflora RO w/ RO right ****Started 3 weeks ago**** I'm starting to get some algae. Lights on for 6hrs. Co2 is ph controlled. Adequate nutrients via EI calculator. Always have used RO w/ RO right before going high tech. Going with co2 I followed the directions and set my ph controller based on 3dkh from aquarium to about ph 6.8. It was fine, saw pearling. Did my first 42gallon wc and noticed my controller was coming on barely if not at all. So I tested my RO water and it's at barely 1dkh. (I'm aware it strips all) RO right brought it up a degree. But RO right is for GH with a trace of bicarbonate that raised it 1. Today I tested my water softened tap and dkh was off the charts. It took 16-17 drops. ??? Can I mix some tap to raise KH? Dechlorinate first? OR add baking soda or a manufactured buffer? I do not want to do crushed coral. Please advise? Thanks in advance.

PS- I have Kent's Superbuffer dkh from my brackish setup. I could use 1/4 the prescribed amount?

[Philly33]

So, I've been able to raise kh and ph to match my tank and RO storage tank by adding a very small amount of baking soda to both. I'm doing this ever so slowly to not stress the fish.

??? Will the baking soda stay stable enough between weekly wc? I read that a kh in the 4s keeps ph more stable