given otherwise great conditions, bristlenose plecos can thrive in something as small as a 10g. that's just about the only kind that can, though. anything with a number instead of a name sure as hell can't (and yes, i've seen them get sold to parents exactly like you said 😦)
what really pets my peeve is adding something like a snail or pleco to "eat the algae" when the real problem is the ecosystem is getting too much light because it isn't on a timer!
