I like to run two filters on each of my tanks, as a back up, and so I can pinch filter media whenever I need it. The more heavily stocked one has a double sponge filter with biomedia compartments underneath the sponges, plus a small canister filter for extra cleaning power. Other tank is a quarantine/grow out tank and has frequent water changes, so a single sponge filter and a HOB with sponges and ceramic rings in it is plenty.
I don't like to run airstones on their own. Why bother when I can attach a sponge filter to that air pump and it'll do the job of an airstone, plus add filtration, and the fish and shrimp love it too? I had an air pump stop working once, so having a second filter on there gives me piece of mind that if one conks out, the other will keep things ticking over. The more filtration, the better, and it's handy when anything goes wrong or you want to jump start a cycle.
