Hi Emma.It sounds fine to me,except if it has tadpoles they will need plenty of algae, plant life,and then insects to eat in order to thrive.Parent frogs do not care much for where they produce frog spawn and whether conditions are correct,when it is a well know fact they will use a puddle if need be!!
If i were you i would remove some of the floating duckweed as it can get out of hand but be careful not to remove any tadpoles or insect life with it.Then i would plant something like a small water lily,providing there is a reasonable depth,and some oxygenating weed.If in the mean time you put in a little fish flake for the tadpoles,then that will ensure their survival.
Other than that a scoop of the bottom every autumn to remove rotting leaves and other bits that can cause pollution,your pond will pretty much maintain itself.It is all about having a balance of enough plant life to attract insects and thus help the amphibians.This will also prevent green water and the dreaded blanketweed,which feeds on excess nutrients not utilised by the plants first.
