In article ,
Earthenticity wrote:
I can't really comment on your case but a few months ago after doing a
large water change, and then a big feed that i had major health problems
in my tank, fortunately in my case it only affected my tiger barbs.
Symptoms were gasping for oxygen at the surface before losing balance...
I remedied by doing another water change, at 11.30pm mind you, closely
watcheed water quality testing ph and ammonia for next hour and adde an
additional airstone.
Next morning all sick looking fish were recovered with only the one
death.......i have no idea what may have caused it but a previously
healthy and long established tank just had a hiccup.


Could ne a few things.

If the pH is suddenly made more alkaline and there is ammonia
in the water it will become quite toxic. Of, if the tank had
very hard water because of topping up with tapwater after
evaporation and a large water change makes the water more soft
by less that half the hardness or the original water, osmotic
pressure on the gills can lead to osmotic shock and death.

If the lack of oxygen is suspected, Untergasser suggests
3% hydrogen peroxide, no more than 25cc per 100 liters
(I'd use half that dose, frankly). This will nearly instanly
saturate the water with oxygen.

I'd suspect the ammonia/pH thing first though, frankly.

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