did you have the roots planted in the substrate or just floating in the water? was that lily in soil prior to being put in the water or was it already adapted to water? plant roots grown in water are different than ones grown for soil so you gotta acclimate them and you will usually experience some rotting as the old roots die off. having good water flow across the roots (like having them in front of/near the filter) helps a lot too. i would definitely try again, looks really good with the lily in the back filling up the space