Interesting, mine are nearly always bottom to mid. The only time they come to the upper is to feed.
Pretty much every other advice I've read says groups of six or more, which is why I went for 8.
So the tiger and the rugby black aren't closely related?
When I previously mentioned upper level, I was meaning fish that swim above the substrate, as opposed to substrate level fish like cory catfish that tend to remain on the substrate. There are species that remain (or prefer to) in the upper level or the mid-level, but all are "upper" fish. [By the way, if you have floating plants, you would find the Tiger Barb more likely to move up to some extent.
Giving minimum numbers for a group of a shoaling fish species is not always clear. [And one has to be careful of the source. Anyone can set up a website and provide "advice," but it may not be reliable. Knowing the source is important so we can trust the advice.] It is always best to have more when this can be accommodated. Some species do show distinct behavioural traits with too few and need more in the group. Tiger Barb is one of these; 8 is usually considered absolute minimum, but 10-12 is better, and you do have the space for this. It likely won't solve the issue though, but that depends upon the individual fish. It is in the nature of this species to be feisty (at best) and very aggressive at worst.
Black Ruby Barbs and Tiger Barbs are both cyprinids, and in the common designated group of "barbs." But fish within such a grouping can be as different as night and day. Neon tetras and piranhas are both "tetras." Being closely related within the same family doesn't govern behaviours or compatibility.
Edit: Essjay posted as I was typing, and noted that the species are in different genera. Which opens the door for me to explain...
The scientific name (genus and species) is the last and most specific in the hierarchy of scientific classification (termed taxonomy). The genus is part of a Family, and the Family belongs to a certain Order; for our purposes, we do not need to go higher than the Order. The Family and Order can each be further divided into “Super” and “Sub” families and orders. Each of these terms includes “clades” or clusters of fish that are phylogenetically related. Phylogeny is sometimes referred to as the natural relationships and is an attempt to construct the history of all life based on the evidence from both living and fossil organisms. When classifications are based on phylogenies we can ascertain (and predict) how that group of related fish function, and since this tells us something about their behaviours and requirements it is of interest to aquarists
Defining “species” is very complex; for our purposes, we may simply consider that the species is the individual fish that is unique from all other fish species. When two or more species are phylogenetically related they will be combined in the same genus. All species in the genus have descended (evolved) from the same common ancestor. This is referred to as a monophyletic clade. This is a relatively new method, only made possible by the discover of DNA in the 1970's, so many of the then-existing classifications might not be completely accurate. A genus which includes species not descended from the same common ancestor is termed polyphyletic. As an example, the genus
Corydoras is polyphyletic because several ichthyologists have determined that the 160+ species are not descended from the same common ancestor, and there are actually nine distinct clades (genera) within the polyphyletic genus
Corydoras. Some day that will get sorted out.
