Bronze Cory - toxic reaction?

Good evening,
A couple days ago I move two nearly adult Bronze Corys (C. aeneus) from a quarantine tank to my 55 gallon community tank. Catching them was pretty easy. The "slow net" technique works well on them. I placed them in a plastic bag with about a pint of tank water and set that in the main tank. After they had set there for a bit I pulled some tank water into the bag and then tilted it so the fish could swim out. I had read that Corys can exude some kind of toxic material when disturbed to I was trying to prevent more of the water from the bag from going into the main tank. The catch was not stressful until I lifted the net from the water and of course that is pretty stressful.

The result on the other tank inhabitants was astonishing. 2 male Pearl Gouramis huddled together in a pocket among the weeds near the bottom. Four Siamese Algae Eaters were no where to be seen. Likewise with five Diamond Tetras. The Corys (now 5 Bronze Corys) seemed to go about their business near the bottom as did 4 Yoyo Loaches. The overall appearance of the tank was no fish swimming in the water above the plants. They looked terrified. Normally when I walk past the tank the fish come to the surface nearest the corner of the tank closest to me, begging for food. (They're shameless in this regard, begging even when their bellies are already distended with food.)

The next morning I did a 20% water change and cleaned my canister filter (which was overdue anyway) and replaced one of the carbon pillows. By the end of the day things were almost normal and today things are pretty much back to normal.

It's pretty clear to me that the two Corys released something into the water that disturbed most of their tank mates. If I ever need to transfer Cory's again I will absolutely not transfer any water with them. But that might not matter if the fish continue to release the toxin once they're released.

One interesting side note is that when I released the Corys into my quarantine tank I also released them with two small Siamese Algae Eaters (purchased at the same time.) In the main tank the SAEs were definitely affected but I didn't notice any unusual behavior in the Q tank. Of course they were pretty shy for the first few days and I ascribed that to new surroundings.

This is interesting. Bronze Corys are a lineage 7 species which are thought not to produce toxins.

Fear can be induced by smelling / tasting a predator. Did the Corys come from a tank with a cichlid or other predator in it?

Ichthys said:

This is interesting. Bronze Corys are a lineage 7 species which are thought not to produce toxins.

Fear can be induced by smelling / tasting a predator. Did the Corys come from a tank with a cichlid or other predator in it?

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The only other fish in the tank the Corys came from were to very small Siamese Algae Eaters. That's my quarantine tank and I usually fill it with water fdrom the main tank (or a mix of RO and filtered tap water.) In other words there is not likely to be anything in the water from the Q tank aside from what is in the main tank.

Can you think of any other possible cause of such a striking behavioral change for the fish in the main tank?

Is it possible they are not actually Bronze Corys? I see a lot of Corys in pictures and at the LFS that look a lot like these but are not (e.g. Green Corys and IIRC there were others.) Is there a similar looking Cory that is known to produce toxins?

If you are moving fish from one tank to another at your home, there is no need to bag them, float them and then release them unless the water in one tank is substantially different to the other tank. And that's unlikely unless you are modifying the water in one tank but not the other.

I used to catch fish in one tank and put them straight into another, no acclimatisation need for tank swapping at home.

As for why the fish were spooked, it could have been a big plastic bag floating about with a couple of fish in. If it were poisoning, all the fish in the tank should have reacted and they should have been gasping at the surface or at the very least irritated and breathing heavily.

If you want to dilute something in an aquarium, do a big (75-90%) water change. Small 20% water changes don't dilute anything and 80% of the bad stuff is left behind.

Good to hear they seem better now though

HankB said:

Is there a similar looking Cory that is known to produce toxins?

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Yes, there’s a few from lineages 7, 8 and 9 that look similar. Brochis (L8) are sometimes mis-sold as Bronze. Lineage 9 produce the most toxins.

And as Colin says, you only need to float and mix fish if the parameters are different. In your own tanks they’re likely to be about the same, so just net them over.

As for why the fish were spooked, it could have been a big plastic bag floating about with a couple of fish in. If it were poisoning, all the fish in the tank should have reacted and they should have been gasping at the surface or at the very least irritated and breathing heavily.

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Gasping at the surface would be a typical reaction to a toxin that makes it difficult to obtain oxygen or perhaps anything that makes them uncomfortable. The best description of this behavior is terrified - some kind of fear reaction causing them to hide. The two male Pearls, for example, are not normally on good terms and find their own place in the tank. For a day they huddled together in a couple different pockets in heavily planted parts of the tank (at the bottom.) The effect lasted far longer than the bag was in the tank, leading me to believe it was something in the water.

Yes, 20% only dilutes the water a bit. And I will go directly from one tank to another in the future. Old habits die hard.

Thanks both for your thoughts and suggestions on this experience.

Do you have a picture of your Corys?

If you can get some very clear photos of the cories from the side and above, it might be worth asking on the CorydorasWorld FB page. Ian Fuller is often there, and there are other very knowledgeable people there who may be able to make a more solid ID on species, because there are similar looking species sometimes mis-sold as bronze cories, as said above.

Would be interesting to learn more, I hope you update if you learn anything more! Glad the rest of the fish settled down again okay, and there were no losses.

I'll work on getting decent pictures.

HankB said:

I'll work on getting decent pictures.

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A camera with a fast shutter speed, and an algae tab dropped near the front of the tank (or a tab that you can stick to the glass) after cleaning the glass inside and out is my usual technique! Good luck Fish photography is never easy....

Fish photography is never easy

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Indeed! Thanks for the tip. I cranked up the ISO on my camera, dropped a couple algae wafers in and wasted a lot of digital film. The other interesting thing is that along with the SAEs and Bristlenose Plecos, the Pearl Gouramis really like algae wafers. (Apologies for the algae on the glass - next water change...) This is probably the best shot of the two big ones. The single one is a (much smaller) one of the trio of originals. It's cropped a bit more so it looks the same size. I can see a difference in coloration.