I've had a 2.5g pico going for a while now, so I know it's possible (and I don't think I'm insane - but some might argue with that
).
However, the tank does not contain any fish. It has only two cerith snails and two blue-leg hermits. I use the tank mainly as a frag grow out and a place to attach mushrooms that would otherwise get lost in my bigger tank.
I have a thin layer of coarse live sand, 43W of compact fluorescent lighting, some live rock, and a large HOB filter filled with cheato. Everything came from my main tank.
I do daily topoffs and water changes, using a mix of water from my main tank and lower salinity RODI mixed salt water.
Honestly, I wouldn't recommend it. It's a lot more work than my 30g tank which, incidentally, also requires daily topoffs, but only weekly water changes. And the lighting was a lot of work, as most fixtures that small are not really sufficient, so I had to build my own.