Congo tetras killing cardinal tetras?

[Oli]

Hi guys, so a bit grim but I just walked past my tank and saw all my Congo tetras going mad like at feeding time. I noticed they were all eating the head of a cardinal tetra. The tetra was still bright blue/red which makes me think it hasn’t died naturally?

The tank has only Cardinal Tetras, Congo Tetras and a Male Pearl gourami?

 

[Byron]

Having maintained two different largish groups of Congo Tetras twice in my 30+years of fish keeping, and having them in with small shoaling fish both times, and from my research on this species some years back, I would be quite certain that the Congos are not actively killing the cardinals. If a fish dies, within minutes most other fish will consider it likely food. This doesn't always happen, but I have seen it.

Of course, there is always another side to this, that of the tank environment. If the environment is not suited to a species, it causes stress, and severe stress can cause behaviourial changes in the stressed fish. Like all animals, behaviour is determined by genetics and environment. Not saying this is the case here, just noting it is a possibility. But more likely (we hope), the fish happened to die and became "food."

The male Pearl Gourami might be the aggressor, another possibility. One of the less aggressive of the medium-sized gourami, it is still a possibility.

It might be instructive for you to pull up a chair and sit absolutely motionless in front of the aquarium for up to an hour...without moving, so the fish "forget" you are there. This can allow us to see the actual interactions going on. If the fish knows you are there, they will often be more on their "best behaviour" because they associate you with food. Sitting motionless for some time allows them to forget you and get back to business.

 

[GaryE]

As usual, I end up seconding Byron. I too have kept congos - they are an old favourite. I also keep cardinals. I doubt congos would predate, but they do scavenge. And there is quite a size difference.

Cardinals like slow, darker tanks and Congo scream out for bright light and very fast moving, highly filtered water. A fast water fish kept in a slow moving tank, or a small tank, will sometimes show weird behaviour. But demolishing cardinals would seem very weird to me.

 

[Byron]

I would offer a different perspective when it comes to light. My last group of 10 Congos were in my 90g tank which had a heavy cover of floating plants (photo below). Red Tiger Lotus with multiple floating leaves (I encouraged the floating leaves because they are so beautifully red/violet when floating under the light), along with Ceratopteris cornuta (Water Sprite) that would after three weeks completely extend side to side. It was interesting that the Congos remained just above mid-line of the tank, but when I did thin out the Ceratopteris, they moved down just below mid-tank where they remained until the surface was again covered, and they would move back up. It was also very noticeable that their colouration sparkled under the dimmer light.

From my profile of this species:
Origin and Habitat: Africa. Inhabits calm waters of streams and rivers in the Congo [Zaire] River basin. Available fish will likely be commercially raised.
Compatibility/Temperament: Very peaceful. Must be kept in a group of 10-12 minimum; can easily become frightened otherwise. The male is the more colourful and a mix of equal male/female is perhaps best, though less of an issue than with many other characins. In spite of its larger size and being an active swimmer, this is a timid fish and should not be combined with boisterous tankmates. Not suitable with any fish likely to nip fins. Often appearing drab and colourless in the store tank, when placed in a well-planted aquarium with a dark substrate and subdued lighting they shimmer with metallic hues of blue, green, orange and gold as they swim. Floating plants are necessary to both reduce the lighting and to calm the fish. Filter current should be minimal.

 

[GaryE]

The breeders of this fish whose set ups I've sat in front of have had moving water in their tanks, with the fish spawning in natural sunlight. There were two guys in our club who sold a lot of young Congos before the farms started producing them. One had a planted tank as a room divider, a 6 foot tank, with some light coming in from basement windows. It was really something.
A friend has caught them in the wild in water he described as small flowing streams/rivers that were tributaries to the Congo. They were far from still, but the tetras were in still and moving water both. Here, I found them looking happier with water movement.

Fantastic fish that were always really peaceful here. That we can both confirm.

They are innocent. I'll vouch for them!

 

[itiwhetu]

I won't be the Congos, something else is going on

 

[Moby]

I‘am currently keeping congos and cardinal tetra with no problems at all.

 

[Oli]

I can only imagine it was a weak cardinal that maybe passed away and was quickly eaten. I can seat the tank in front of me from my sofa and have never seen any aggressive behaviour in the tank.
Thanks guys!

 

[Oli]

I have just fed the fish and ten minutes later, I noticed again the Congo tetras going nuts all eating another cardinal tetra. I am thinking maybe the cardinal are getting accidentally taken out during feeding time because there is so many of them going for the food at the same time and the congos always go crazy at feeding time. Is there a solution to this? Maybe I will simply have to rehome the Congo tetras?

 

[Byron]

If possible, a video of the feeding might help us determine what is actually occurring. How much is beeing fed (video will help with this too), and what food exactly?

 

[Colin_T]

Separate them.

Congo tetras are too big and boisterous to go with smaller tetras like cardinals and neons.

 

[Oli]

I’ll try and get a video tomorrow at the next feed. They are just fed tropical fish flakes. A decent pinch that is eaten in under 2/3 minutes. All the fish immediately rush to the top of the tank and they are all in and amongst each other going for it.

 

[Oli]

Separate them.

Congo tetras are too big and boisterous to go with smaller tetras like cardinals and neons.

It would be a shame because they are beautiful, calm fish that swim back and fourth across the tank all day. They never show any aggression or chase smaller fish. Come feeding time however they go absolutely mental and I think they will be unintentionally hurting the cardinals