I've been ponding for about 20 years, and I think Sean is right on: use any fertilizer you like, just do it. I am a perfect example of forgetfulness when it comes to fertilization and it aint pretty.
That being said, I've noticed that adding a pond tab every now and then is very beneficial because of the micronutrients. I'm a skeptical person so I'm not 100% sure of anything, but it seems that micronutrients round out the nutrients available to the plants when kept in pots. I do have some lilies that have jumped their pots (I'm lazy) and are perfectly healthy- growing in a clump on the bottom of the pond getting their nutrients directly from the muck and water.
I think this topic is really interesting because I've lately been obsessed with how healthy things are "in the wild". Why are wild cats (cougars, tigers, etc) healthier in the wild than they are in captivity? Nutrition. Why are plants healthier in their native habitat? It is because they have carved out their niche in nature and get all the nutrients they need. We have to correctly nourish our water lilies for the same reason that a zoo has to correctly nourish a cougar.
None of that would be possible without the sun.
But what do you think about the micronutrients? Is it that much of an issue? Do any of you notice a difference when you add micronutrients to the equation?