Maybe take out the plastic plants and maybe remove the substrate. If you can't do that, try adding root tabs or some sort of nutritions material under the gravel. This way you can plant stem plants instead of plastic ones, which are much better and prettier for your tank.
I highly recommend ludwigia palustris, as it is fast growing, pretty, easy to take care of(just have fish in the tank lol), and so easy to propagate. Only need 1 plant and some time, and you can just snip parts off and replant them. Just make sure there's enough nutrients in the substrate to plant them.
Another one is java moss, which also takes off. However, try to tie it down and keep it away from duckweed as it will seriously mess it up.
EDIT: Get floating plants too. You can get really anything, just avoid duckweed at all costs. It's a nightmare.