I have a 10 gallon tank which until recently held 6 glowlight tetras and 4 small cories. On Saturday, I introduced three endler's guppies. Sunday afternoon, I noticed that one of the endlers had whitish fins and all 3 were hanging out near the surface. I didn't think much of it because the people at petsmart had handled them quite roughly, so I thought the fish might have been injured.
That night, the endler with whitish fins was dead. The other two seemed okay, but by the next morning they were dead. At that time I noticed that some of the tetras had slightly ragged looking fins. I've had these fish for a while, and they are not fin nippers, so I became concerned that they might have fin rot. The cories appeared fine except that one had a small whitish area at the edge of its fin. I did a 30% water change and hoped that they would be okay.
That night the cory with the whitish edge of his fin was dead. At that point, I added API Fungus Cure to the tank. None of the other fish appeared to have worsened. The next morning, I found 2 of the other cories dead and that night the final cory was dead. The weird thing is that the other three cories appeared perfectly fine apart from the fact they were dead.
With the cories gone, I added aquarium salt to the tank in hopes that it would help the tetras fight the disease. As of this morning, several of the tetras still have ragged fins and the whole school is hanging out in the back corner, but they otherwise appear fine. The water quality is stable 0-ammonia, 0-nitrites, 10ppm-nitrates. It is somewhat acidic, but it always is and up until now these guys have all been fine.
Any ideas what's wrong with my fish or advice on how to save the remaining 6?