Mystery guests

The pond has some mysterious new occupants. At least four small brown fish have been spotted swimming happily amongst the plants and tadpoles. Nobody has the slightest idea where they came from. We can only assume they were put in by another tenant on the site, perhaps because they didn't want them in their own pond, or perhaps as an anonymous gift for all to enjoy. We know that people love to come and sit by our pond and they're very welcome to enjoy it.

We guess the fish must be eating the pond weed because that has cleared almost entirely. Does anyone know what sort of fish they are and how should we feed them?