Yeah I'd add more, stems in particular as they'll grow quickly for you.
If you want a bit of a game plan, the Alternanthera reineckii on the right and the Hygrophila corymbosa(?) on the left could be definitely chopped in half and tops replanted right now, you could get a mix of Rotalas and maybe some Limnophila hippuridoides for behind the left hand woodwork ( I think there's a few stems of H. polysperma there at the moment), some Hygrophila pinnatifida would be nice to attach onto the front most woodwork and maybe some Blyxa japonica for some of the spaces up between the crypts. Also, that giant mound of fissidens fontanus to the left could be divided up into smaller portions and either pressed into the spaces between the rocks or tied/glued to rocks and scattered around, it'd grow out fast then. Any other spaces you could jam a bit of hydrocotyle tripartita sp. Japan for the time being, if you can tolerate a bully.