What is wrong with these fry?


A reader is concerned about his baby guppies who have bent backs, and asks if euthanasia is the only option. Fish expert Peter advises...

Q) I was wondering if you could advise on why my baby guppies have bent backs. I’m quite new to the hobby of late but used to keep tropical fish about 30 years ago. The guppies are in a planted 85 l tank with some Cherry shrimp. I originally had a male with two females (I’ve since moved the male into the main tank). One of the females only gave birth to one baby and then died, but the other had seven, and while she’s still doing great, I’ve noticed six of the youngsters have bent spines. So, I moved the ‘normal’ fry to the bigger set-up but left the mother in the small tank with the infected babies. I’ve done a lot of research, and I’m wondering whether this is fish TB. If that’s the case, will I need to cull them — and if so, what’s the most humane method of doing so? Would I also have to do same with the mother? Both my tanks are cleaned and maintained every week. I feed Tetramin food, and the water is spot-on, apart from being a bit hard.


A) PETER REPLIES: You are correct in that fish TB, caused by Mycobacterium marinum and other Mycobacterium species, is one cause of spinal deformities, particularly if the fish become infected during early development. This bacterial disease can be passed from an infected female to her young, and some or all the young will be born infected. Spinal deformities in fish can also be caused by other types of infections, as well as nutritional deficiencies, and by certain chemicals in the water, such as high levels of certain heavy metals.

Other possible causes include genetic disorders and environmental problems during the fish’s development, such as low oxygen levels and high water temperatures, although some of these may be more influential on the egg development of egg-laying fishes, than fry development in livebearers. My best guess would be either TB or a dietary issue, but I can’t be 100% sure. How are the other young and adult guppies: do they show signs of TB, such as becoming emaciated, or trailing white faeces? I’d also check that your fish food has not gone past its use-by date. If it is TB, then this disease is very difficult to eradicate, as you have probably read. Assuming the mother looks okay, then I’d leave her by herself (ie. Not with the male) and consider culling fry that have the bent spines. I would advocate the use of clove oil (available from a pharmacist) to do this.

Using clove oil to euthanise fish

Clove oil is poorly soluble in water. One way to help it dissolve is to mix a measured amount of clove oil in a small amount of ethanol. An alternative method is to dissolve the clove oil in a small volume of hot water. Whichever method you use, you then further dilute the dissolved clove oil in some aquarium water until you reach the working strength. As a general guide, a dose of 10-15 drops of clove oil per litre of water is required to euthanise a sick fi sh. It’s best to use water from the fish’s home aquarium to dilute the clove oil as we don’t want to further stress the sick fish by exposing it to a sudden change in temperature, pH, chlorine, etc.

Make up the clove oil anaesthetic solution in a small clean vessel, such as a glass jar, and ensure it is mixed thoroughly. There should be plenty of depth to cover the fi sh. Before adding the fish, double check your maths to ensure you have not over- or under-dosed. You can then gently add the fish. Position it away from bright lights or other potential stressors. Closely monitor the fish during anaesthesia. A hand lens or magnifying glass may help in the case of small fish such as guppies.

You may see some erratic activity at first, but the fish should soon settle down and maybe roll onto its side as it falls into deeper states of anaesthesia. You should see the fish gradually lose body and fi n movement and the gill beats will become shallow and maybe erratic. After a few minutes the fish should be completely motionless and not respond to being touched. Given the poor solubility of clove oil there is a real risk that the fish may still be alive, due to insufficient dosage. If you still observe any signs of life after say 10 minutes of immersion, add another dose (at 10-15 drops/litre) of clove oil. You may even need to repeat again, with a third dose, if necessary.

It might seem tempting to add a massive dose of clove oil at the very start, to ensure you have enough dissolved, but sudden exposure of a conscious fish to a very high dose of anaesthetic can be very stressful.