Broken aquarium filter?

Yesterday we discovered that the aquarium filter is not filtering very much water. The water should cascade out of the spout, but only a low dribble is coming out now. It may be broken, or it may need cleaning.

The goldfish themselves are seeming pretty lethargic. I changed a quarter of the water this morning, so I hope that helps.

I'm also wondering if the lack of light is affecting them adversely. They are in a pretty dark area which only gets real light for minutes at a time throughout the day. Their lethargy may be them sleeping, now that I think about it.