I asked the people at Water Garden Gems in San Antonio about a means for
controlling Aphids on Lily leaves. Here is their response:

Good afternoon,
Aphid's can be controlled by using Safer Soap and spraying the pads. Be
careful not to do it in the middle of the day, try to do it in the evenings
so that the sun does not shine on the pads. Safer Soap is a safe product to
use in and around the pond and can be purchased in most nurseries or Wal
Mart in the gardening section.
Thank You
Connie

I did find it at Wal-Mart. It is called "Safer 3-in-1" and comes in a 24 oz
spray bottle. The cost was $5.

Jerry in San Antonio