I have a Whisky Bareel pond going now. It's amazing how much "stuff" finds it's way in there. Lot's of leavers, flowers, twigs, dead floating bugs.
Regular maint. is mandatory on small container ponds. Mine is about 25gl. I do a 6gl water change every other week. and a monthly filter cleaning (rinse out the sponge).
My is only planet with aquarium plants planted in clay pots.
OP - I suggest to try a few other Aquarium plants other than the java fern.