Its your Ehiem Liberty filter that is causing the problem because its a Hang-On-Back (HOB) type. HOB filters are totally not good for a planted tank, remove it and you will find that your CO2 will dissolve in the water better and your pH will drop because of the CO2 and not anything else. HOB filters will dissipate CO2 to the surroundings cuz they have a waterfall style of water return to the tank. Less surface aggitation = more CO2 in the water = better plant growth. Change your filter to an internal filter or a canister filter and your plants will grow much better