Fish are fine in captivity, as are maggots. No pain, no problem. Look at their neurology - I'm not asserting something in a vacuum.

What causes them pain is injury, anoxia or hunger, but not having their "freedom" impinged upon. As long as they are swimming (fish, not maggots) in something big enough to provide the oxygen, which they are in the pedicure thing or they'd be flopping about trying to breathe, instead of eating people's dead skin off their (the people's) feet, they are fine.

Maggots require even less.

I'm not "imagining" that they don't care. I can conclude it.