The onset of winter presents many challenges to pond owners, not just with respect to the safety and well-being of their fishes, but also for that of the plants. In general, in places where you have “real” winters with thick snow, icy-cold temperatures, etc., most tropical plants do not “make it” through the winter especially if they are left in the pond, so they should be removed as soon as the water temperature drops below 60 degrees Fahrenheit.
Depending on their variety, try using them as house plants. The conventional wisdom about Water Hyacinth, Pistia, as well as Water Lettuce is that they take a lot of work and money to keep alive all winter, so it is best to consider them as “annuals” and to just replace them each spring. But if you don’t want to consider this option, perhaps you can “winterize” their habitat by “putting on” a light and making it a warm, frost-free place (much like a greenhouse) with at least a 16-degree-Celsius temperature (that’s about 61 degrees Fahrenheit), so that they can “hibernate” (yes, some plants actually hibernate during winter).