The last two posts both mention difficulty growing floating plants, so perhaps some general observations may be helpful.
First, they are fast growing plants, and that means they need good light (intensity and spectrum) and nutrients to balance. Being at the surface, intensity should not be a problem (though it can be), but spectrum might; aquatic plants need red and blue light to photosynthesize, and red is the more important; having green in the mix does improve plant growth considerably. So in spectrum a light in the 5000K to 6500K range is what you need. Light that is higher than 6500K (which means more blue and less red, and perhaps less or no green) is not going to be much good long-term. The duration has to be balanced by all nutrients, since photosynthesis follows the Liebig Law of Minimum...plant growth (photosynthesis) is governed by the least sufficient of the 17 required nutrients, not by the excess of any nutrient (always assuming light intensity/spectrum to be sufficient).
As for nutrients, floating plants have the aerial advantage, meaning that the leaves being on the surface can readily assimilate CO2 from the air rather than the water, and this process is about four times faster for the plants so they usually get all they need which cannot be said of submersed plants. Other nutrients may be lacking, or not quite sufficient to balance, and a comprehensive liquid supplement should provide this. It has to be comprehensive...including all nutrients in the proportion plants require, so the plants and not algae benefit.
The covered tank is often suggested not only here but elsewhere, but my floating plants do not have difficulties in my covered tanks, and while they are certainly not sealed air-tight, they are well covered.
Floating plants usually take time to settle in to a new environment, so be patient.