Thanks. Live plants help process nitrogen. Also bacteria on all surfaces including substrate as you said. It sounds like in your aquarium those things together can handle your bioload. If that is whats really going on, I wonder how common this is.
"I try to keep an extra filter in there when I think the old one's getting past it's time to go, leave it in for two weeks and then remove the old one. (If I'm going to switch out filters.) Although I hadn't done that for the past few times, and I never saw a blip. (Plenty available apparently on the Bio-Wheel, plastic floating plants and gravel apparently.)"
If your filter has a biowheel, that is where the significant majorityof the biofiltration is supposed to happen. It is a large surface area for bacteria and it keeps highly oxegenated water flowing over this surface. I would guess that's why you saw no blip when replacing the other filter the last few times. The other filter is primarily for filtering particles out of the water. That double-up-for-two- weeks idea is a good one though,for filters without biowheels.