I have a Fluval 303. It is a great filter, just make sure you use the right media for the filters (sponge, ceramics, pellet carbon) and you should have crystal clear water. 30-40 gallon tank is what I is well rated for.
The recent model is the Fluval 304. The 304 is square vs the round 303 and the water moves across sideways in the 304, not up and down like the 303.
I prefer the 303 over the 304 myself. But I have only been using it for a few weeks, so keep that in mind, however I have read mostly positive reviews about the 303.