Another idea would be the just pull the UG between the L. sphaerocarpa and the L. red and replace it with a different green plant say hygro purple or similar....... food for thought.
Yes, I use to have the Elatine hydropiper growing there.
Starougyne purple would look nice there. But Starougyne would yield a better color combo/contrast vs Elatine. But Elatine hydropiper is a neat plant