Well then... you know how I sometimes say things can happen to the best of us?
Guess what: Something happened to me last night and I had to go through an emergency protocol this morning.
Yesterday during the waterchanges I stirred up some mulm, rather a minor amount. So I thought. This morning my filter was almost completely clogged. Fish were breathing fast, but not dangerously so, luckily.
I got the filter out completely (usually I would only do the media compartment, but I thought, while I'm at it...) squeezed the sponges, replaced a matt of
filter floss and cleaned out the pumphead.
And all that before coffee and work. Yuck.
Current status: All fine, no losses, everybody breathing normal again.
Memo: Clean out the pump more often.