I don't have any experience with this plant, but some general web searching suggests most lillies, including Nymphaea pubescens are heavy root feeders, thus would benefit from root tabs.
Also, are you dosing anything other than Flourish comprehensive and Flourish Iron? NPK are all important, and very little of these nutrients is supplied by Flourish.
N and to a lesser extent P can be gotten from fish waste if your water changes are not overdone. However this has its own drawbacks, and may prefer doing large W/C's and adding nitrate/phosphate after the water change (it is also less stock-dependent this way).