A couple weeks ago we bought a brand new filter,, with a very different filtration system. This weekend, I noticed that one of the fish was looking pretty ill: top fin was somewhat cut up looking, and he was moving very slowly. I took this as the motivator to get the new filter installed.
I had realized that things were bad, but not *how* bad: essentially, this was like moving from a swamp to a clear brook. The fish seem more active, and in general, the tank just *looks* healthier.
So hooray! Now I just need to make time to go get more fishies.