Hey, i have 2 types of tetra in my tank, 8 neon and 4 black. they are very similar, and i expected them to shoal together but they dont, in fact the 4 black tetras have split and swim around solo instead of in a 2nd shoal. does anybody know why this is or how i can help them be more solcial ? my tank is heavily planted with some bogwood and a cave.
Black neon tetras
Hyphessobrycon herbertaxelrodi are a significantly different species than neon tetras
Paracheirodon innesi, and shoaling usually doesn't occur across species. Fish use a lot of visual cues to stick together (a large reason fish will sometimes have a distinctive spot or splash of color on the tip of one fin), and since the two groups don't look like one another, they won't shoal together until stressed -- i.e. if they feel enough of a threat or stress that shoaling with un-alike fish seems like a better idea than not shoaling at all.
The last point leads to a second point, that shoaling is generally a defensive position. My fish tightly shoaled for about a week or so, and after they got used to their new hole almost never tightly shoal anymore. The large amount of plants and caves are a large reason for this -- they feel comfortable enough that a quick hiding place is nearby if needed that they don't have to revert to shoaling as a defense anymore. Shoaling really and truly is a strategy based on each individual fish thinking "well, some of us are probably going to get eaten, at least if we're in a large enough group, chances are pretty low that I'll be the specific one eaten." If you've given them a comfortable enough home, they don't have to revert to that mindset anymore. I'd personally be happy that they aren't shoaling -- it tells me that they feel very safe in the surroundings you've put them in.
