Fertilizer and ponds . . .

Now I know that it's a no-no to allow drainage from surrounding
landscape to contaminate a pond, due to the risk of accummulated
fertilizer and chemicals.

However, this is a different question:

I have plants in pots that come from a lake-margins environment and
which live in nature with their toes in the water, but nothing else.
In my pond, they sit in pots on a ledge in the pond that's about 2
inches under the normal water line. I water the plants occasionally,
and they seemed satisfied with the routine. I never fertilized for
fear of contaminating the pond w/ fert runoff.

Unfortunately, after a year of doing well in this location, the plants
are beginning to show signs of nutritional deficiencies --- namely
potassium and nitrogen (I am an experienced gardener and have had good
success addressing signs of such deficiencies before, so I feel fairly
confident in making this assessment).

I'd like to fertilize the pots with something like Potassium Nitrate,
but of course, that would gradually make its way into the pond, and
then ---yikes!

I know that water lilly fertilizer (in "pill" form) is used for
totally aquatic plants by burying it in the media, and that the pots
have no holes in the bottom, so the fert is more likely to stay in the
pots and around the roots . . . but the fish regularly mess with the
stones in the pots of the underwater plants and throw them about, so I
wonder whether this fertilizer gradually makes it into the general
pond environment.

So I'm confused: How can fertilizer for waterlilly pots be OK, yet
fertilizer for mostly-above-water plants not be OK?

Thanks!