Your tank will have to cycle again every time you add fish. Before you added fish you said your tank was fine, then you added fish and your ammonia and nitrites spiked... that would be your tank trying to cycle again. If you still are losing fish, I would advise waiting for a few weeks before adding any more fish. Adding the air stones and more water circulation was a very good idea. Your fish will be happy once your tank finally finishes cycling.
Does this answer all your questions? Your post was kind of long, so I may have missed one or two.
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Phishie

"An ounce of prevention is worth a pound of cure."