Cardinal tetras becoming aggressive?

[Desktop Aquatics]

I have a 17gal with 8 cardinal tetras and a pair of apistogramma. The tank has been setup for about 4 months and I have had my share of dead cardinal tetras.... started with 12 down to 8 and then 7 as of last night. The first 3 were dead the first week in the tank from my LFS and the last 2 have gotten sucked into the filter. Now my once peaceful school of 8 has become an aggressive group of 7 that are picking at each other over night since the 8th one died. Is this common in tetras? I know larger schools dissipate aggression but just because one is gone now they are all aggressive? wanted to hear more thoughts on this as I haven't dealt with this before.

yes I have a sponge over my intake, they still get in sometimes not sure how....

and water parameters were better than usual

ph-6.8

ammonia- 0ppm

nitrite-0ppm

nitrate-10ppm

       

[laritheloud]

My first thought -- is it breeding behavior? Have you noticed any chasing of females by the largest, most aggressive male? I don't have cardinal tetras, but I have Diamond Tetras, and the 'alpha' male at the top of the pecking order will beat on the lesser males when he's in a breeding mood.

Edited July 30, 2021 by laritheloud

[Desktop Aquatics]

I have no experience sexing tetras at all I will have to do some digging... it seems to be everyone picking on everyone its very strange

[laritheloud]

Tetras can be wild. I think sometimes they can fool you into thinking they're semi-aggressive. Part of the issue, I've found, is when they're the 'biggest fish' in the tank. They have nothing that will encourage them to school or temper their behavior. My diamonds, for instance, were really railing on each other (and always breeding) before I added honey and thicklipped gouramis to my 29 gallon tank. Now that the gouramis are in, they are much more tame and school more often; but then again, their numbers have also 'accidentally' nearly doubled from 6 tetras to 11 due to all the breeding they do in my tank.

[Fonske]

On 7/30/2021 at 12:38 PM, laritheloud said:

Part of the issue, I've found, is when they're the 'biggest fish' in the tank. They have nothing that will encourage them to school or temper their behavior.

Interesting.  I have ten cardinals, one diamond-head neon and six green neon tetras in a tank with one balloon pearl gourami (he is a gentle tank boss). The cardinals ignore every other fish including themselves, they don't school and don't pick on any tankmates. The lonely diamond-head acts just like a cardinal. The green neons, on the other hand, vigorously chase each other all the time.

I guess there might be some species-dependent minimum number required for non-aggressive behavior, even in the presence of a larger fish.

 

[Daniel]

On 7/29/2021 at 11:10 PM, laritheloud said:

My first thought -- is it breeding behavior?

That's my thought also.

[laritheloud]

On 7/30/2021 at 2:09 AM, Fonske said:

Interesting.  I have ten cardinals, one diamond-head neon and six green neon tetras in a tank with one balloon pearl gourami (he is a gentle tank boss). The cardinals ignore every other fish including themselves, they don't school and don't pick on any tankmates. The lonely diamond-head acts just like a cardinal. The green neons, on the other hand, vigorously chase each other all the time.

I guess there might be some species-dependent minimum number required for non-aggressive behavior, even in the presence of a larger fish.

 

I have only Diamond Tetras and they grow to be on the larger side for tetras. My biggest males sit at around two-and-a-half inches. I definitely agree there's some variation between species. I don't want to indicate to anybody that I think these fish are semi-aggressive because my diamonds are truly 'peaceful' and non-combative with every fish that isn't their own species. They have never bothered the other fish in my tank, and they're actually rather meek around them. I'd also call them skittish fish, because they're the first to hide when I stick my hands in the tank.

But man... their social behavior with one another can get very rough, especially during breeding.

[Jungle Fan]

I've kept cardinals on and off since 1968 and I've only ever seen them become aggressive in very small groups when something had happened, and for a short while your group might be down to just a handful.

On the other hand I've seen apistos go after everything that moved when they breed. Are you sure your cardinals killed each other, or are you guessing, it sounds to me that what you see might be stress nipping from being pursued by a bigger killer all through the night.

Apistos aggressive breeding behavior, and their taste for shrimp are the two factors why I keep Bolivian rams in my 75 gallon tank instead of Apistogramma cacatuoides red, or Apistogramma macmasteri. Don't get me wrong apistos are great fish, I love them, they are normally great tankmates, but when they breed it's no more Mr. Nice Guy, and they get down to business as I learnt the hard way a few decades ago.

Cardinals do best in larger groups, the bigger the better if you want to experience their true schooling behavior and see them less stressed. I know that books advocate for 7- 8 minimum a lot, but they really start being more relaxed around 15 in my experience. I keep 35 in my tank and it's a joy to see them parade up and down lengthwise in the tank, and around the driftwood.

[Desktop Aquatics]

On 7/31/2021 at 4:54 PM, Jungle Fan said:

I've kept cardinals on and off since 1968 and I've only ever seen them become aggressive in very small groups when something had happened, and for a short while your group might be down to just a handful.

On the other hand I've seen apistos go after everything that moved when they breed. Are you sure your cardinals killed each other, or are you guessing, it sounds to me that what you see might be stress nipping from being pursued by a bigger killer all through the night.

Apistos aggressive breeding behavior, and their taste for shrimp are the two factors why I keep Bolivian rams in my 75 gallon tank instead of Apistogramma cacatuoides red, or Apistogramma macmasteri. Don't get me wrong apistos are great fish, I love them, they are normally great tankmates, but when they breed it's no more Mr. Nice Guy, and they get down to business as I learnt the hard way a few decades ago.

Cardinals do best in larger groups, the bigger the better if you want to experience their true schooling behavior and see them less stressed. I know that books advocate for 7- 8 minimum a lot, but they really start being more relaxed around 15 in my experience. I keep 35 in my tank and it's a joy to see them parade up and down lengthwise in the tank, and around the driftwood.

oh no, I think maybe you misunderstood me. I know how they all died it was not due to aggression, the cardinals are just picking at each other now that there are only 7. When there were 8 they were a solid school never saw aggression. The male apisto does occasionally lunge at a cardinal but I have yet to see any actual fin nipping besides the tetras on each other. 

[Jungle Fan]

@Kylec as I said the larger a group you'll have the less of that type of behavior you'll see. Small numbers cause stress, they feel less secure, and just as with people under stress, and duress you'll see lashing out from some. Getting more cardinals should solve that problem.