Try glass worms. They eat it gladly, and the food can live for a day with Your fish. I feed my paros every second day, becouse there are always some uneaten larvae.
With white worms it’s better to underfeed slightly, than overfeed, since the overdose will die and decompose.
The pH will go down with time, all You need to do is let the leaves rot.
Do You have some kind of filtration? You probably are unable to have any Lemna, becouse of biofilm on the water. There are small filtres with skimmer, it might solve the problem.
Beautifull C. vendtii on the first photo. It’s a shame they haven’t found their way into this tank. They would make an ideal cover.