The advertised flow rate is probably measured with no media in the filter.
- Make sure it's assembled properly. I've heard of people doing things like leaving the carbon packet in the plastic bag and wondering why they're not getting any water out.
- Make sure everything is clean. I know algae and other stuff really slows down my AC
HOBs, even if it's just a film.
- Look into upgrading the impeller. You can do this with some filters. I've done it with two ACs and it worked well.