Don't feel bad about the Ludwigia 'Pantanal.' It, along with a large portion of the plants we grow, are not as easy to grow in water around GH 14-16+ than in GH 5-9 or below.
I have found the Rotala genus, especially macrandra and sp. Nanjenshan, to be very tempermental when grown in hard water. They do grow, but growing them in water around GH 5-6 is effortless by comparison.
I have managed to observe this recently in my 10g, where the sp. Nanjenshan began doing poorly when I went from 50/50 tap/RO mix to 100% tap water.
Having jungled tanks between both Miami's softer tap water and Chicago's harder tap water, I have observed this reaction several times.