I'm trying Nymphoides cristata. Just arrived yesterday, look good: big roots under the surface. I'm even trying one planted (completely submerged) they sent me so many. (aquariumplants.com)I saw these advertised for use in a pond - I wonder if any can be used in a tank.
Lemna trisulca
Nymphoides crenata
Nymphoides cristata
Nymphoides indica
Utricularia vulgaris