If it really is difficult to get a fertilizer tab in the pot, then it is problably root bound or very close to it. You can re-pot that one in a larger pot, adding fertilizer at the same time, and likely not see much of a set-back due to what's called "shock" from the re-potting. Just keep the root mass intact and add the extra soil under and around the mass, keeping the crown at the same level (not covered with new soil.)
You could also divide the waterlily if there is more than one crown growing, which what I'd probably do. But that usually does take a little time for the plant (now, plants) to get back to growing and blooming. If you still have a couple of months left in your growing season, this is the way I'd go.
What I read, but have yet to do, is when using the Nutricote prills, sold as Red Diamond in the garden shops, is to pull the plant, root mass intact, from the pot, add the prills, and put it back in. Then you can keep it going until early spring and do the usual re-pot and/or division.
Mike S.
Spring Hill, FL
