Tank water cleanliness

I've recently purchansed a tinly little 11 litre (about 2.5 Gallon I think)
tank for 2 goldfish. It came with a little filter which obviously isn't
doing a great job as the water gets cloudy after about 3 days, after a week
it's quite murky - still see through, but not pleasant to look at!

I understand that 2 fish may be too much for 11 litres, so is there anything
I can do to keep their environment a bit cleaner?

I feed them 3 times a day, and generally not more than they would eat over a
3 min period (as per several web searches have suggested)
I also refresh about 10-20 percent of the water in a week, however this
doesn't help enough.

Any suggestions would be appreciated, as I want to keep the two fellas
happy!