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Apologies for posting a non catfish question here but I was hoping the many decades of combined experience of Planet Catfish members might be able to shed light on this subject.

I look after a large community tank for a friend of my mum's, it about 1,200l and contains a great mix of species, not ideal but they get along - Silver sharks, tinfoil barbs, clown loaches, Gibbiceps, bristlenose, bleeding heart tetras, flying foxes, rainbowfish and discus.

I have been performing 40% waterchanges on roughly a three week interval (not ideal but it keeps the nitrate under 20mg/l) for the past few years with no issues. I cleaned the tank yesterday and left with all the fish behaving normally and apparantly in good health.

Today I get a call to tell me that the silver sharks (and only them) are in distress. I went over and had a look and sure enough they not happy, of the six, four are lying on their sides breathing rapidly, the other two are swimming 'normally' but also breathing heavily. None of the other fish are breathing faster than usual nor are any showing any symptons of disease. The affected sharks do not have any redness around the gills nor are there any apparant signs of either bacterial infection or protozoal (velvet).

I added about 200g of rock salt to the tank in the hope that this might help with any osmotic trouble, as well as about 500ml of Sera Toxivec.

Water conditions are as follows:
26degC, pH 7.5, kH 8 (degrees), Nitrate below 20mg/l. I did not have an ammonia test kit with me but I'd be surprised if it is at leves that would affect the sharks but not any of the other fish (esp the discus). The tank is well aerated and has two old Eheim bucket filters on it.

So, does anyone have any experience with Silver Sharks being more susceptable to any toxins or water conditions? I don't expect the four most affected fish to survive the night but at the very least I would like to work out what has happened so that I can prevent it in the future.

I would be thankful for any help anyone might be able to offer.

It sounds like there has been a sudden drop in dissolved oxygen level. Balantiocheilos are particularly sensitive to that.

Was there a sudden rise in temperatures (ambient or otherwise) recently? Are the filters working normally?

I don't think temperature played a part, the tank is very stable in this regard and for it to have warmed sufficiently to cause problems and then cool back down to the regular temperature overnight seems unlikely given it is in a climate controlled appartment.

However low oxygen levels does seem the most likely cause of distress to the fish, I guess it dropped just below the tolerance of the sharks but not far enough to bother the other fish. Perhaps the tap water used in the water change had unusually low dissolved oxygen levels for some reason, or there not enough gaseous exchange at the surface? It does seem unlikely given both filters were pumping strongly and there are two large air stones in the tank. But stranger things have happened. There is no CO2 on the tank btw.

Anyway I returned the next day to fish out the four deceased sharks, cleaned the filter that was next due for it and reduced the water level enough to ensure there was a good splash from the filter returns. The surviving sharks were still breathing heavily and not interested in food, but I've not received a call to say they have succumbed. Fingers crossed they will recover.

Thanks for the help Silurus.

I hope they recovered!

I had issues like this when my 55gal suffered a bout of Old Tank Syndrome and my nitrites spiked overnight - suddenly killing off half the tank. Make sure you are testing no2s in addition to the no3s. No2s prevent oxygen from getting into their bloodstream and into their brains. I think it's called black blood syndrome or something like that.