for that size container your fish will stand more chance of survival if you
can have running water of some kind - e.g. a small fountain. You also need
to get something to cover about a third of the surface - either some kind of
floating plants (fairy moss or duck weed would do - they spread very quickly
but you should find it easy enough to scrape off the extra in a tank that
size) or a miniature (not "small" but "miniature") water lily. The "scum"
sounds like algae which may disappear once your pond achieves a "balance".
Covering part of the surface will help as the algae will not have enough
light to grow as much then. Also when the oxygenators grow a bit they will
take more food out of the water so will starve off the algae. You also need
somewhere for the fish to hide from the sun. I'd leave it a bit longer -
more like a month.
Hayley