I personally don't use any ferts for my lillies (or any other plants for that matter), but one thing I've found beneficial is to re-pot them at least every 2-3 seasons. I usually do this at the begining of spring. at that time I usually do add the fert tabs, but I never add anything once they are submerged.
there are devices that help with getting the ferts to the lillies, but they may or may not be reliable. they use a tube which holds a pellet of (proprietary) fert tabs and uses a syringe type method of injecting the tab into the soil. if most ferts are left above the potting elements they can contaminate the water column which is why I mention they may not be reliable (if you can't make sure the tabs are deep enough).
HTH