You could do either. Have to say ours have had very few flowers this year so maybe the season has something to do with it as well. Does it normally flower well? Just wondering if perhaps the water is a bit too deep for the variety?

it doesn't really need it. The temperature variation is part of the problem, the water didn't warm up steadily and this will have an effect.

2 of mine flowered later than normal and one was very shy this year. I haven't split mine for 3 years and they are growing outside their pot but that isn't a problem as their roots absorb nutrients direct from the water and don't need mud/soil/grit except as an anchorage point.

Do they get enough sun? Water lilies need 6 hours of direct sun every day in order to bloom.