Cardinal fish aggression

[M Stein]

I've had a Banggai for about two months in my 15 gal, and decided it was time to get a school. Unfortunately, the store only had one in stock, so I only got him for now.
Doubly unfortunately, the first Banggai decided to harass the second one to death.

My question is would I have better luck introducing two new ones rather than only one and was the aggression due to the being the same sex?

 

[X-37B]

I put 2 small ones in my 20g nano cube. So far they are peacefull. Other fish is a sixline.

 

[M Stein]

Here's to hoping it stays that way.
They also seem to be closer in size then mine were.

 

[Dave1993]

i believe the only ones that school are pyjama might be wrong tho

 

[M Stein]

I've seen Banggais school as well.

 

[Mr. Mojo Rising]

The tank might be too small for 2+ banggai's IMO.

Schooling is a defensive mechanism, the idea is "I hope you get eaten instead of me". Its rare to see schooling in an aquarium, especially a very small aquarium.

 

[ISpeakForTheSeas]

I don't know if this applies to Banggais, but, as I understand it, with most fish species that school in the wild, they will only school in aquariums if there are enough of them (the minimum number I've seen is 7 specimens [6 for some freshwater fish], generally I've seen 9-11+ recommended). If there are just a few fish (2-5/6) then they kill each other off. If there are more fish, then - for one reason or another - they accept each other with just a few squabbles and no/minimal killing.

That said, I agree that a 20 gallon tank is probably too small for a school of Banggais. In fact, some people might say that a 20 gallon is probably too small for one Banggai. Regardless, if you're determined to try adding more, you might want to see if you can do just a pair of them instead of a school.

Edit: 15 gallon for the OP - I put 20 after reading 90's Reefer's post. The same applies though. (Glad it's working well for you 90's Reefer!)

 

[Risky Fishniss]

I just bought 3 Bangai Cardinals for my 40 gal tank. I was told to buy at least 3 as they like to school.

 

[DaJMasta]

Two banggais of the same sex will fight eventually, even if they are young enough not to be doing it now. If you want to keep more than a pair, i'd look toward more of a group of 5+... Unless you have a lot of space, a smaller group will likely have a pair and a third wheel or similar configuration where not all will survive, but with a larger group, it's harder for them to single out one and less likely any do real damage to each other.

I've read that in the wild, they have been seen in schools of hundreds, so the upper limit is how much stocking you can have in your tank.

 

[nereefpat]

I've seen Banggais school as well.

Only as juveniles. They pair off when mature.

 

[i cant think]

I've had a Banggai for about two months in my 15 gal, and decided it was time to get a school. Unfortunately, the store only had one in stock, so I only got him for now.
Doubly unfortunately, the first Banggai decided to harass the second one to death.

My question is would I have better luck introducing two new ones rather than only one and was the aggression due to the being the same sex?

Kauderni will only group as juveniles in-between long spin urchin spines. This is purely for safety and shelter but as they age they will split off into pairs.

 

[Risky Fishniss]

Ok, thanks for all the information. I guess we will see what happens.

 

[M Stein]

Thanks everyone for your advice.
It seems I'll have to stick to just one for now. (Unless I can sex them)

 

[DaJMasta]

The easiest way to sex them is to look for the number of vents on their underside, there are some descriptions of how, but until they are adult sized it's quite difficult to see the vents in my experience. Your best bet is probably taking an up close picture of your fish against a high contrast backdrop and seeing if you can make it out - though you'd need the same on the one you're adding.

That said, if you have a plan for removing and rehoming it, you can also just try one and see. They will know in the first couple of minutes whether there's going to be a problem, and generally one will get chased into a place where it's far easier to catch, though just putting a clear acrylic trap in a place they like to stay is a fairly reliable way to trap them.