Threads been hijacked! Haha. ...
:fun2:
Sometimes things develop a life of their own ...
Maybe we should start a new threat (or a mod can split it off)
... There are many species that should be alone unless there is enough tank space for them to have their own territory. So it depends on tank size and species.
That is part of my point: species that are usually found in pairs or social groups in the wild should be kept as such in the tank.
if you can't keep them as such in your tank then you shouldn't keep them until you have a tank where you can.
But if you can keep 2 angelfish or butterflies of different species you can also keep a pair of one. I kept such fish as Pygoplites diacanthus or Chelmon rostratus successfully in pairs in rather small tanks (750 liter & 500 liter).
Dwarf angels are for sure the easiest angels to be paired up and some species should only be kept in pairs or groups or they go crazy over time (Centropyge argi for example).
And there is hardly any excuse for intentionally keeping single specimen of such easily paired fish like clownfish, mandarins, pipefish and seahorses.
I also kept pretty much all my dottybacks, jawfish, marine betas, gramas, gobies and damsels in pairs. In general, if you know what to look for, you can tell male from female.
One of the local stores here in Fremont has a pair of hawk fish, and I have seen people keeping another species - the long nose hawkfish - in pairs. That shows that these species can be kept in pairs as well.
I only kept 2 tangs (as a beginner) as singles but know of several aquarists in Germany that keep at least Paracanthurus and Zebrasoma species in pairs or groups.
Of course there are some fish where it is hard to keep them as pairs because they either get way too big or only pair up at certain times for spawning. Those are species I would avoid keeping in a tank.