I would like to recommend earthworm castings,a natural,organic time
released plant food,however it (and compost,fish emulsion and bat
guano,etc)is full of beneficial bacteria,which may cause an algae bloom.
If your lilies are in a pot,try adding some to one pot at a time next
time you repot the lilies and watch for algae.Its available at some
nurseries,mail order,or on the web,you might have to look around to find
it.
Or grow your own and feed the worms to your fish as a high protein
treat!