These are tuberous species and so require a summer dormancy. If I
am not mistaken your summer is about July. At this time the
plants will die back and you should remove them from the water
and let them dry out completely. Begin to water them in Autumn
and they should re-emerge from their tubers. If you keep them wet
over summer the tubers will rot.
Stick to an open compost 50:50 peat:sand does the job.
Cheers
Terry