The problem is that soap clings to things really well, and the amount of soap that can be fatal to fish is very miniscule. I wouldn't want to take a chance with getting soap in my tank.
This is the way that I clean my fake plants:
Pull out from the tank.
Put them in a plastic bag.
Place bag in a large plastic bin.
Place bin near the side of the road on Monday nights.
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Sorry. I have been slowly been getting rid of my plastic plants, and I just let them get covered in algae prior to disposing of them.