fluval 304 filters are fine, mine works a treat.
they draw the dirty water up through the bottom casing and trap the dirty water in the filter wool inside and then expel the cleaner water out through the top. look closer and you can see the jet of water coming out of the top.
you need to stop the filter (take the plug out of the socket first) and regularly clean the filter wool by washing it or just renew it, it will be full of fish waste and algae