GH needs to be high enough to be reasonably sure you are getting some magnesium, or you can add Epsom salts to the water for magnesium. Your 75 ppm is adequae for calcium. KH just needs to not be too high, where too high is around 10 degrees of KH or a bit less.
You have high light intensity in that tank, so you absolutely need pressurized CO2 to let the plants grow at the rate that light is driving them to grow. And, the CO2 needs to be at the optimum concentration, too, plus being well distributed around the tank by having good water circulation in the tank. If you don't have CO2 that would explain what you are seeing - the stem plants are hogging the minimal CO2 that is naturally available.