Does my fish have flatulence?

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Dr Peter Burgess gives his advice to a reader who is concerned their fish's flatulence could be fatal.

Q) I have a small group of Hemiodus gracilis, all around 3in/7.5cm in size, which relentlessly patrol the front of my tank. Recently, I noticed that one of the four had developed what I thought initially was a swimbladder problem, desperately swimming downwards in order to stay with the rest of the group in midwater. The buoyancy problem is a daily occurrence that develops only after the tank lights have been on for 5–6 hours. When the lights are off, the fish appears fine and swims normally with the others. But I’ve noticed that this particular fish appears to be actually ‘eating’ the oxygen bubbles released by the plants during photosynthesis — hence the link to the lights being on. Presumably the air in its guts is voided somehow after the lights go off and buoyancy returns to normal. Flatulence might be amusing, but in this case could it ultimately prove fatal?

PETER DALLAS, VIA EMAIL

A) DR PETER BURGESS REPLIES: Flatulence is quite normal in fish, and they will naturally accumulate small amounts of gas in their guts which is eventually voided via the vent (anus). But we rarely notice it in our aquarium fi sh.

However, it is abnormal for a fish to accumulate so much gas that it causes positive buoyancy (a tendency for the fish to float upwards), affecting swimming function. Where we do see positive buoyancy in fish, it is commonly due to abnormal gas accumulation within the swimbladder(s) or the gut.

Excessive gas in the swimbladder can be a sign of an infection or physical distortion, and once established it tends not to resolve itself, becoming a permanent issue, as sometimes seen in fancy goldfish, where their abnormal body shape can cause deformation of one or both swimbladder chambers.

In short-term cases or when it comes and goes, as in the case of your Hemiodus, then, as you rightly conclude, it is more likely to be gas accumulation within the gut. Excessive gas can be due to a gut infection or poor diet but it can also be due to ‘airswallowing’ such as when fish feed greedily at the water surface and take in air with the food. Given your affected fish has taken a liking to ‘eating’ oxygen bubbles, then I’d agree that this seems the most likely explanation for the daily buoyancy problems experienced by this fi sh. It probably perceives the shiny, moving bubbles as prey.

I’m not sure what you can do about it, though. I suspect you are feeding the correct amounts of food, but you might try giving a little extra to see if the fish is bubble-eating simply because it is hungry. Perhaps you could try redirecting the filter outlet, so the plant’s oxygen bubbles get swept away – although that could excite the fish even more. Other than that, you may have to live with this strange behaviour-related condition. Whether any harm will come to the fish if it continues with its bubbleeating addiction, I cannot say. Who knows, it may live to a ripe old age!