i keep my angler (black unidentified species) in a 1.5 gallon pico and feed him one damsel every week as well as 2 ghost shrimp every day
hes about 1" long and seems as happy as he culd be, comming to the front of the tank to beg. hes been in there for over 6 months.
it is a temporary housing situation tho, as he will be moving to a 2.5 as part of my frogfish display (him, Antennarius striatus, Histrio histrio, and hopefully a white collar) housed independatly in a series of picos.
i feel that, due to the fishes mainly stationary lifestyle, as long as water quality is maintianed (frequent water changes, lots of natural filtration and circulation) they can be housed in incredibly small tanks.