I didn't mention Salvinia previously, as I was thinking more of substantial floating plants (keeping in mind your comment about filter current...Salvinia will congest in my experience), but Salvinia is a lovely plant, with its silvery-green leaves that almost shine from above--where one seldom sees it, lol.
Floating plants settle better in some tanks than in others; I have 7 tanks, all have floaters (I consider floating plants mandatory for most fish species, plus there is the incredible benefit they bring to water quality), but it is still interesting how one species will do better in one tank than others, and another species is better in a different tank, etc. I have have basically identical lighting, comparable fish loads, same fertilizers in all tanks. This only proves once again that no two aquaria are identical, and is one reason diagnosing problems can be so tricky. I think you might have good luck with Water Lettuce in less warm water.
Some attribute failing floating plants with the light "burning" them, or the lack of air flow with a covered tank. There may be something in the latter, but if you have fish that may jump then covers are essential. I have had my 70g empty of fish for a couple months now; I intend re-scaping this tank and presently it is just running with the plants but no fish (some fry appeared a few weeks after the fish were removed, and they have now been moved). The Water Lettuce in this tank absolutely took off; the roots reached 2/3 of the way down into the water, and it spread like a weed, and the individual plants grew several inches across. Half the cover glass is off, with only the back half under the light to prevent water splashing from the filter or water changes onto the light tubes. I am intrigued if this open top has been a factor. I also had the heaters off, until I found the fry and the weather turned colder. The fish room is heated to around 70F, so the tank would have been at this temp when the Lettuce went wild.
But picking up on Ch4rlie's comment about nutrients, this is very true. I have experimented with Flourish Comprehensive Supplement (the basic complete liquid), Flourish Trace (trace minerals or micro-nutrients, though not all of them), Flourish Iron, and Flourish Potassium. I found that floating plants tend to do best with the first three of these, spread out over the week. When I was dosing just the Flourish Comp twice weekly, algae became an issue in some tanks, and going to just once with the Comp and adding Trace solved this. We can discuss these more when we have a better idea of the setup.
