My airlift tube was clogged or something: no water was going up into the plants from the reservoir. I futzed with the pump, trying to get some result through pressure change. No luck.
I picked up more tubing at a pet store and decided I would install it the next day. Until then, I’d just water the plants manually and hope for the best.
The next day, everything was working again.
I’m still going to replace the tubing, but now I can put it off until after Alchemy and midterms.