03-03-2010, 11:57 AM
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I am more inclined to think the problem is fin nipping by either the neons or the Black Skirt Tetras [I'm assuming your "blackstripe tetras" are actually Black Skirt Tetras]. Both of these tetras, as indeed almost all tetras, can finnip when placed with tempting fish like angels that are sedate and slow cruisers. Also, keeping the tetras in small groups (4 is not sufficient, should be 6 or preferably more) can encourage this behaviour, as can too small a tank but in a 55g that is not or should not be an issue.
The other thing is the water parameters. A pH of 7.2 is not particularly high on the alkaline side, but it is certainly higher than any of these fish prefer, and that means there can be ever so little stress to the fish, and stress can cause behaviour modifications.
In other words, what I am saying is that a number of negative factors that may be slight individually can, when combined, cause changes to the behaviour of certain fish. The solution here is to remove either the angels or the tetras. If you have a spare tank, removing the angels for a couple weeks and monitoring their condition would be useful. Fish that are subjected to bullying in even small doses will be stressed, and poor health follows down the road.
Having said all that, I am prepared to bow to the advice of someone with more knowledge of disease in case this is a disease that I am not seeing.