The other thing you might want to try is a tiny amount of pelleted granular slow-release fert in a little twist packet made of newspaper. I use this on my waterlilies and lotus in my little fish pond. Works as well as those expensive tabs made for water plants. No ill effects in 4 years (although it is submerged and poked down into the soil in my plants)
Foliar feeding with soluble fert pretty much guarantees the fert will wash into the pond unless you don't get rain or dew.
Or, what about putting the plants in baskets, and changing them out with others on a regular basis (maybe monthly?) so you can fertilize them when they aren't in the box with the pond water flowing through. Two sets of plants on a rotation schedule? Hm, it sounds like a lot of trouble, but would reduce the fert addition to the pond water.
