Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Larger schools of fish look more impressive than a school with too-few fish. You just do not get that real schooling action with less than a dozen, and closer to 2 dozen is better.
What tank is large enough for 2 dozen of even the smallest fish? You could get that many Ember Tetras or Dwarf Rasboras (there are several tiny species) into perhaps a 29 gallon, but smaller tanks are just not great at handling overstocking.
By the time you get to 40 gallons there is enough water that you can handle 2 dozen of something small, and a bit of room for some algae eaters, bottom feeders or another small school of something. And the 2 dozen can be some slightly larger species like any of the smaller Tetras (Neons, Cards, Rummynose...) Harlequin Rasboras or any of several other fish that size.
You could divide that 2 dozen into 2 schools of a dozen (which are barely schools, but possible), but then you need to select the right species. 2 that are too similar will swim in the same area but usually won't school with each other, so all you have is a mess of fish. So select species that look different and act different. Select a bottom school like Cories or Loaches (gotta stay with small fish), or an upper school like the smallest Hatchets. Or choose 2 species, one stays relatively high like Guppies or Danios, and a lower school like Cardinals.
That is what I mean by "right on the edge of working with 2 schools"- it can be done, but very carefully select the species you want.
A larger tank has more volume so 2 full sized schools can be at opposite ends, or mingling then separating, and will look distinct, even if they are similar. (Unless they are so alike they school together, and who would buy 2 species that look that much alike?) Or you can still stick with one main school, but make it larger fish like Congo Tetras Red Line Barbs, Phantom or Bleeding Heart Tetras, then add some of the larger Cories or Loaches (from Zebras to Kubotais depending on the tank size), or go with some of the larger upper area Danios or Silver Hatchets, and there is enough room for a decent sized school of each.
Avoid the 'Noah;s Ark' tank. "I'll take 2 of these, and 2 of those, and 2 of..."
I hope that people are enjoying fish for their behavior, not just the colors. Seeing a good sized group of fish, properly schooling, flowing back and forth, over and under branches, splitting up then coming back together... is very impressive, but it takes the right quantity of fish to achieve that. It just won't happen with half a dozen fish, and barely happens with a dozen.
Fewer species, and more of each species is better when it is a social species.