Water Sprite and Duckweed are my choice for floating plants.
You have no fish in the tank that would destroy plants so you either got bad plants to begin with, or there was a major change in water temperature or chemistry between where they came from and your tank.
If the plants came from a cold water tank or pond and went into a tropical tank they can rot.
If the plants came from water with a high pH (above 8.0) and went into a tank with a low pH (below 7.0) or vice versa, they could die and rot.
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Did you buy the plants from a pet shop or online?
Plants sold online can take several days to get shipped to your house and this can affect some plants, and it will certainly weaken them and make them more susceptible to temperature or water chemistry changes when you get them.
If you get plants online, try to have them shipped overnight and don't buy them when the weather is really hot or really cold.