Your tank was established, and then you added 2 fish, so it would need to cycle again (although it's possible that it was the oxygen). The cycling after the fish were added would not be as bad as starting new, but the system was only able to handle whatever you had in there previously (be that no fish or one fish). Your fish tank will always cycle again with the addition of even the smallest fish. It could have a been a combination of the two (oxygen and cycling). The proteins that were on the surface indicate the low oxygen, but your fish would not die from that alone once transferred to your other tank.
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Phishie
Site Coordinator

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

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