I did grow S.repens quite nicely 4 years ago in pretty much the same setup, and according to my log from then, I am also dosing pretty much the same.I find that s.repens are quite difficult to adapt and grow, mine is doing better since I add more NPK and Magnesium, but I lost 90% of it the first months...can't see why tropica classified it as an 'easy' plant?!
But it sure looks like some deficiency. Perhaps the water from the waterworks has changed its level of Mg. I know for sure they now have lower KH, so there might also be lower Mg. I can not see that in the official measurements from the waterwork, but they also did not show the change in KH.
I will wait some more days, and then try to up the dosing. You know, I am trying to act slowly and only change on thing at the time, even though the instinct it just to act right away - Xim, thanks for all your advice!