A bit strange that kH is different between the two tanks.
I am guessing that you started both tanks with a bunch of emersed grown plants. If that is the case, they do melt while transitioning to submersed growth and dump nutrients and crud into the tank. In that case, water changes do not help that much with accumulation of organics on the substrate and hardscape. The way to overcome this transitional stage is to keep cleaning: remove decay from substrate, clean the glass, rub algae off affected leaves with your thumb, trim off melting leaves. With stems, once you get 2 - 4" of submersed growth, pull the plant, cut and replant the top.