Need help regarding water lillies.. urgent..

Hi guys,

I recently purchased a lavender coloured water lilly, less than a month ago and it also gave me a couple of blooms after buying.. But that didnt last long, the plant started turning black and i started pinching away the discoloured leaves thinking that it may spread to the entire pond.. but slowly all the new buds also started turning black.. im just too worried that the plant may die in the pond.. we tried to change the water and i have also added a little fertilizer to it hoping to revive the plant.. please help me.... :drowning

 

yes we have put in some clay mud as suggested by the vendor.. its is still not recovering..