New frogbit dying

Hi everyone. I have a small wildlife pond which is pretty well established but has too many nutrients (if that's the right terminology) so the hornwort got out of hand and was joined by some duckweed. I got on top of the duckweed and took out a good bit of the hornwort as advised then recently added a small frogsbit in the hope it would grow and "use up" some of the excess nutrients and also hide ugly plastic planters. I've had it about two weeks and it seems to be dying, the leaves are almost all brown. Any ideas why this is happening? Don't want to buy more if its going to die again. No fish in pond but usual "bugs" quite a few snails and newts. Leaves don't look eaten/damaged other than the discolouration. Thanks in advance