If you want them to float properly, each individual plant needs to be separated from the clump and put onto the water surface. They'll quickly cover the tank, 1 plant deep, then if they aren't thinned out they'll grow several cm deep which blocks the light from the lower plants. I had to throw away handfuls almost every week to make sure there was some exposed water surface.
I have red root floater now and that grows as fast as the frogbit did. I have to throw away handfuls of that weekly as well.