So now we see the problem with using ambiguous terms like "big" and "barely adequate".
Saying a 12" fish is ok in an 18" wide tank is a fairly broad statement. What kind of fish is it, and how active is it? How much more growing has it got to do? What else is it being kept with, and how aggressive/territorial is it?
12" fat lazy bichir in an 18" wide tank; no worries.
School of fast active fish like 12" tinfoil barbs in an 18" wide tank; not so good.
Breeding pair of 12" mi-devils; fine.
Community of ~12" CA cichlids; pushing it.
Community of 12" CA cichlids where many still have considerable growing to do; potential time bomb.
All IMO of course....
