11-29-2011, 09:48 PM
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Thank you so much for your very detailed response! I will look into returning the Black Skirts. I have noticed they do like to be the bosses of the tank.
Again, I appreciate your help so much. Thanks a ton.
Originally Posted by Byron
There are several issues at play here; I'll begin with some generalizations before getting to your specific situation.
Shoaling fish need to be in groups for several reasons. First is unquestionably the "safety in numbers" for the fish. They expect to have hundreds of their own species around them, so not having this will be stressful. Second, depending upon the species, they may have a "pecking order" within the group. Angelfish and discus are well known for this, but many other fish such as Cyprinids and characins also do this. Third, which is related, they may have a distinct social structure within the group. Loaches are very high in this aspect.
Lacking a group causes stress, and stress weakens the immune system causing health issues and disease that would otherwise not occur. But stress is now known to do more. A while back I posted a link to the first scientific study that has been done on this and it conclusively showed that aggression is heightened when the group is less than 5. They used several tetra species, angelfish, and I can't remember what others. Fish with normally aggressive tendencies, such as Tiger Barb, got even worse when kept in small groups. And some otherwise peaceful fish such as neon tetra became quite surprisingly aggressive when kept in just 3 or 4 fish. So it does have an effect.
This brings me to your particular fish. The Black Widow Tetra [another common name for Black Skirt] is by nature inclined to be nippy. It should never be kept with sedate fish for this reason. But when kept in groups less than 6 it definitely shows increased nippiness within its group and to other species. If you check our fish profile, it mentions this. [Profiles are under the second tab from the left in the blue bar across the top of the page, or if the name is identical in a post it will shade as it did here, and you can click the name to see the profile.]
Species vary in all this, and sometimes individual fish can be "different" too. All we know is that the inherent trait is programmed into the fish through natural evolution, and it is safer to assume it will eventually manifest itself and provide accordingly. I would suggest you might consider returning the Black Skirts and consider other species that will manage better in your 15g in an appropriately-sized group.
The red X-Ray is presumably a colour variety of the Pristella Tetra. I did a quick search and most sites consider this a "Pristella" with one source using the scientific name Pristella maxillans which is presumably a corruption of Pristella maxillaris, as this is so far the only described species in this genus. So we can say this is a Pristella Tetra. A group of six would be best. It is very peaceful, so you could have some very small fish in with it, like the Ember Tetra, plus some substrate fish like a group of corys (another shoaling fish, minimum 3 or 5 work well). This would be a more peaceful and likely less problematic tank than keeping the Black Skirts.
Hope this helps.