I'm seeing a lot of juvenile leaves, which usually means lots of growth points sharing the rhizome's energy. Have you thinned the rhizomes since you planted them? Hardy water lilies should be growing along the surface of the planting media and splitting into multiple tips over time. As a rule of thumb, I remove half the rhizome growth points every year, and everything that exceeds the pot. Even with big pots I repot entirely at least every four years. Your mileage may vary, of course.
Do you fertilize? Water lilies need a lot of nutrients to perform. If you have rich, organic growing media and/or fish in the system you may have sustained nutrient input to your lilies. Otherwise, they aren't going to perform well without something like pond tabs.
