So KH is 4 degrees, I can't tell the color of the pH vial, it could be 7,4? Check the low pH as well.
Once you establish the pH, if there is not enough CO2 in the water (maybe it is lower as 7,4 currently?) you can try having the outlet of the filter under the water surface for a couple of days, then check the pH again if it is lower, that would be better for the plants.
Once you establish the pH, if there is not enough CO2 in the water (maybe it is lower as 7,4 currently?) you can try having the outlet of the filter under the water surface for a couple of days, then check the pH again if it is lower, that would be better for the plants.