|You mention feeding Cheerios until the bio filter "catches up" in the spring.
How do I tell that my biofilter has indeed caught up?
Is it a certain temperature, build up of "muck" on the pebbles, a chemical test?
Your biofilter is "caught up" when the nitrogen numbers (Ammonia, Nitrite and Nitrate) are all at acceptable levels.
Ammonia - Trace or less
Nitrite: Trace or less
Nitrate: Under 100ppm
A cycled filter has a musty smell to it, and the media will have a plain brown slime on it. This plain brown or greenish slime will be composed of autotrophic and heterotrophic bacteria which use nitrogen from the water. These are the organisms which reduce the nitrogen to those safe levels. This slime coating is called BioFilm and is EXTREMELY important in your filter, and on the tank sides. Also check out information in this site about BioSeeding. (Look it up!)
Temperature affects the rate at which the Cycle progresses and the rate at which these bacteria will reduce nitrogen. This is why Cheerios are good - when the bacteria are slower in cold water - the Cheerios provides minimal nitrogen for the bacteria to be inundated with. So, you're feeding, but the fish aren't suffering with high accumulated nitrogen in cold water.