If those lights are sitting right on top of the tank you have high light intensity, high enough that you need to use pressurized CO2 to keep the CO2 concentration in the water high enough and consistent enough to meet the growing plants' needs. DIY CO2 will not work.
CO2 (in PPM) = 3 * KH * 10( 7-pH ) works only for water that has nothing in it to affect the pH except carbonates (bicarbonates) and CO2. Tap water very rarely meets that criteria, and aquarium tank water even less so.
With the amount of light you have you also need to dose a complete menu of nutrients, N, P, K and trace elements. None of those, dosed correctly, affect the pH. Aquarium salt should never be used for a planted tank.
Tap water may contain lots of CO2, which lowers its pH. But, once in the aquarium, the CO2 dissipates and that causes the pH to rise. The CO2 fluctuation that this causes is also a trigger for algae to start growing.