Any of the hardy type water lilies will work. Just don't get too many depending on your pond size. Too many covering your whole pond surface can cut off needed oxygen for the fish. I have a Water Hawthorne that has just started doing well this year and I like the little white flowers it gets. Also like Parrots Feather, Azolla (a tiny floating mass of reddish green plants. The Sedges in pots do well and help suck up the fish nitrogen fish waste really well. Pretty grasses too also help and look very nice. Yellow Flag Iris is nice too but can become invasive unless you keep it in check. I love Ferns planted around the edge if you have some shade from larger plants for them. Hornwort is a nice one too. It stay beneath the water and just floats freely or as mine did, attached it self to some airline tubing and seems to like that secure feeling. Have lots of other plants growing around the pond edge too.
Sedge grassy plant on the left
Parrots Feather
Azolla growing in a tub pond for safe keeping, otherwise I dole it out to the fish sometimes or they will mess it up too much. Not sure if the eat the whole plant but maybe the roots.
Ferns
Hornwort
Water Hawthorne
Water Lilies (Hardy)
Misc.
Creeping Jenny
Lots more like mini cattails, Creeping Primrose, Watercress, and others let you know what they have also.
