What caused my barb’s whirling behaviour?

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Dave Hulse answers a reader worried about her Rosy barb.

Q: I have an established 200 l community fish tank. I recently added 20 Cardinal tetras and the following morning one of my otherwise healthy Rosy barbs started whirling uncontrollably in the tank. I have isolated this fish and am treating for internal parasites, as I read it could be associated with this, but so far there has been no improvement.

I tested the water and there had been a slight increase in ammonia to 0.25ppm. I am doing daily partial water changes to help the tank adjust to the new bio-load. I don’t want to medicate the main tank as I have some shrimp and I also don’t want to risk damaging the filter.

What do you think has caused this in my barb? It was fine before adding the tetras. I did not isolate the new fish and added them straight to the main tank. Could the new fish have brought something in with them? I’m now worried that my other fish might also become ill.

HEATHER REID, VIA EMAIL

A: Dave Hulse advises: Loss of coordinated swimming leading to an erratic swimming pattern can have several possible causes. From your question, I assume that this Rosy barb is the only one showing the symptoms you describe. Are all 20 new Cardinal tetras accounted for?

You report a slight ammonia rise in the tank, but this is still very low and unlikely to account for the symptoms you describe. Again, if this barb is the only one showing symptoms, ammonia toxicity is really unlikely here.

There is a parasite that causes a condition called ‘whirling disease’ that you might be alluding to from your reading. It’s called Myxobolus cerebralis, and it’s a parasite of farmed salmonid fish, however this, or any tropical relative bought in by your Cardinal tetras is incredibly unlikely to be the cause of the problem due to the timescale. Myxobolus, and others like it, parasitises the cartilage of developing fish. The disease does not infect in such a short space of time.

There are other wasting diseases of tropical aquarium fishes that are much more commonly seen such as fish TB or microsporidian parasites. Alongside the general wasting these infections cause, erratic swimming is often a symptom as the swimbladder is often impacted leading to abnormal posture, and muscle wasting leads to improper locomotion. If either of these are the case, your Rosy barb will also be looking very thin with dull colours. In the case of both TB (caused by infection with various species of Mycobacteria), or the microsporidian parasites (such as Pleistophora or Pseudoloma), there is no treatment. Isolation is essential as infectious spores are liberated from the cadaver, so tankmates must be protected. Again, none of these would have been due to the introduction of the Cardinal tetras, the timescale is far too short, as these chronic wasting conditions do not  strike overnight.

If the Rosy barb is otherwise in tip top condition, the erratic swimming could be due to an obstruction in the mouth or GI tract. Is it possible the fish ate one of the incoming Cardinals? Is the fish eating while in isolation?

Whichever of these causes is at play, the isolation you have already done is excellent. Here you can observe closely and see what progresses, if the fish continues to go downhill, with worse swimming and posture and losing more weight and colour, then this is definitely a wasting disorder of some sort. Be very careful not to contaminate your healthy fish with water from the hospital tank throughout this period.