I am using a Aqua-Clear 500 on a Nano with foam but no skimmer no skimmer. So far Nitrates and other water parameters havent been a problem. But I have multiple Foam inserts, and change it out every 5-7 days without fail, and wash it out thoroughly and allow spares to air dry. I also do 10% water change religously too.
I dont think using foam is such a bad thing, but I think becuase its often out of sight and therefore out of mind, I think they often get neglected for far longer time periods than they should and that this is where problems begain to arise. I think that Procrastination and Neglect are the real culprits.
Just, my opinion though...