11-12-2011, 06:55 PM
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I concur. A sudden and significant pH change is the likely issue here. And a smaller-volume water change, even if twice a week (rather than one larger) should resolve the problem. This would be the simplest and safest solution.
While it is true that most fish can tolerate pH changes, they have to be relatively minimal or over a longer period of time. There is a diurnal pH fluctuation in all planted tanks, usually no more than .4 or .5 of a degree, and spread out over 24 hours (lowest pH at the beginning of day, highest at the beginning of night). Fish have no problem with this, as it replicates natural pH fluctuations in many habitats. There are also seasonal pH changes caused by rain and flooding, and this is believed to be one inducement to spawning. But rapid and significant pH fluctuations can cause severe stress and worse.
The reason is that water is constantly passing through the cells of fish, what is termed osmosis. The hydrogen or hydroxyl ions [these are what determine pH] in the water entering the fish will tend to shift the pH of the fish's blood accordingly to make it the same. The fish must control this, or it will risk severe life-threatening issues. This requires energy, and over time weakens the fish more and more. This fish will naturally gasp for more oxygen, but also the gills are closely connected to the pH regulating process. Slower pH changes that are not excessive allow the fish to adjust more naturally.