Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
A 180 is large enough that if the species want to behave differently they can.
Cards tend to stay away from the light, for example, so are more often found in the lower parts of the tank.
I am not sure if Rummy Nose Tetras (there are actually more than one species) have that same tendency. If so, then you will have 2 schools that want to live in the same general area. The tank is large enough for this, if that is what you want.
If you had 2 schools of lets say 2 dozen fish each, and a few of each decided they did not want to stay with the school you would still have a couple of nice sized schools.
In a smaller tank, not capable of holding that many fish you would only be able to try to have 2 schools but only half a dozen fish each. If a few did not want to be part of the school then there might be only 3-4 fish trying to be a school. You lose the effect. In a smaller tank get a dozen fish all the same species, then, if a few did not want to hang out with the crowd you would still have enough to be a school.
In either tank you can get other species that like to inhabit different areas in the tank.
In a 180 I would think about a couple of dozen Hatchets or Glass Cats for the upper area, and 2 dozen Cories for the bottom. Each of these groups are also a school, so the 180 would have at least 3 schools.
In a 20 the bioload limits the choices, but to have more social groups of fish I would try a small group, perhaps a pair of Rams or Apistos for the bottom, a school of the smallest Hatchets for the top and a mid-tank school of Cards or something. Alternate: A school of Dwarf Cories for the bottom, and a lone fish like a Betta or Honey Gourami for the top with a mid-tank school of small Tetras, Danios, Rasboras or other.
If you must have 2 schools of mid-tank fish in a 20 gallon, then get a couple of dozen fish, but they should be the smallest such as Dwarf Rasboras, CPDs, or other tiny fish. That way you can get enough of each species to behave like a school.