I have 2 in ground ponds and a deck container pond. The small pond
and deck container have a lilly, some small version of waterlettuce,
WH, hornwort, anacharis and duckweed in them. They also each have 2
koi that are under 3" in addition to tons of mosquito fish. The fish
leave the plants alone... I have achieved 95%+ plant coverage on both
ponds... but, if I put my fingers in the water the fish come up to try
a taste them.

On the other hand, my large"er" pond (1350 gallons this week since we
finally had a cold week that in the mid 80's and evaporation is not so
bad) I have 5 koi, 3 GF (would love to give the away now) and 2
comets. I have the same plants plus parrots feather and a bunch of
marginals that the fish cannot reach. The veggi-filter is fine, but
if any WH or duckweed make it to open water then they are doomed. The
fish is the large pond are under 16" and they do root around in the 4
lillies. I just make sure to keep peagravel on top of the soil and I
haven't had any troubles. The hornwort and anacharis get bound up in
the lillies and the fish love to hang out in it.