My fish is laying on the fake plant and barely moving. What's wrong?

There are several causes that can provoke these signs fro water imbalances to specific diseases like swim bladder ones.
This problem requires a quite radical but logic approach.
Begin by submitting a water sample for quality testing. Most pet stores will do this for free or a nominal fee.
High ammonia levels, PH imbalances may be responsible for that. Oxygen concentration and quality of the filters should be determined as well.

If water quality is good and there are no major imbalances, your fish should get examined to look for any sign of bacterial/fungal infection and eventually water should be treated.

Please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.