If you cannot identify the problem, who knows what you can do to ensure they are safe.
You can bleach or potassium permanganate dip the plants; grow them in a fauna-free environment;grow them emersed. Randomly treat them with chemicals.
Personally, plants are pretty cheap, far cheaper than risking future luvestock,... So I'm with Fishorama above, just ditch them and get more and grow those.. it can be incredibly cheap if there's a fish or plant club locally to you . You don't say where you are.
If it's something incredibly rare, I would split depending on the plant, grow some emersed, dip some in different chemicals and grow separate without fish .
Personally, for me, the effort needed to keep the plants is going to be far more than the health of new fish is going to be worth.