04-04-2013, 02:53 PM
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Getting back to the original issue, Stoke88, I would take a very different approach. In my opinion, this has nothing to do with oxygen in the tank, or you would see all fish gasping at the surface. One fish doing this when the other fish are behaving normal [I assume from your data they are] is clearly a specific health issue with the one fish.
I have seen this, in a tank of 130+ fish, where one tetra will remain hanging at the surface. Sometimes it recovers, more often it dies within a few days. I've no idea what might cause this. I have one pencilfish in my 90g behaving like this as we speak; it is also off-colour, a bit washed out. Is or was the angelfish showing normal colouration, with clearly-defined patterning?
Increasing water movement, aeration, or the plain old water change will often help the lone fish. But from this we cannot jump to the (erroneous) conclusion that there is some sort of oxygen deficiency in the tank. This is very highly unlikely in any aquarium with sufficient and working filtration.
Keep an eye on the fish doing this and let us know what follows.