I concur. With respect to the existing glowlight danios...I have found there are times when a group of a species may be reduced in size through disease or old age, and adding more young fish may not always be possible. While i cannot speak for the fish's thinking, I prefer to let the remaining fish live out their lives where they are used to things. As JD said, some species will readily form some sort of attachment to a related species, though this does not always occur; I'll return to this momentarily.
Concerning acquiring another species...definitely acquire the necessary numbers, and placing too few of a shoaling species in a new environment can cause real trouble. There are now scientific studies proving that aggression increases in all species (those studied included angelfish, tetras, rasboras I believe) when there were fewer than five in the group. But, one cannot take "five" or "six" as some sort of magical number; the point is that fewer than five cause considerable aggression but this does not mean to say that five or six did not impact the fish's stress to some degree. The "minimum" number can vary from species to species, and it is always wisest to provide as many as you can manage, and if the present aquarium size and inhabitants prevent adding sufficient numbers, then avoid that species.
Back to the reduced numbers of the present group. I have had many cases over the years where my group of say 10 or 12 fish became reduced with old age deaths. In the majority of cases, the survivor fish seemed OK. But in a couple of cases, the survivors suddenly turned aggressive, and seriously so, imperilling the other fish in the tank. When this occurs, the survivor fish must be removed; there is no point in recklessly endangering the other healthy fish, as stress from an aggressive bully is very severe to fish, just as it is to children.
Byron.
