If I had to choose, I'd go with the lilies rather than the lotus, myself. The water lilies are more dependable, bloom better and best of all they cover (shade) the water better than the lotus will. This keeps it cooler, helps prevent so much evaporation and as previously discussed it will also cut out more sunlight to help prevent so much algae from growing. Oh, and also helps hide your fish from marauding birds too.
You can find hardy water lilies that will stand the cold very well and they come in a fabulous array of colors.
Get a nice water lily established in there this summer, then maybe next spring you'll want to add your lotus. They don't sell the tubers for lotus at this time of year anyway, in my experience. You need to order them in February or March.
