Ignore the alkalinity, yes it makes ammonia more toxic but ammonia is a short term issue as soon as fully cycled.
Throw away the dip strips ASAP as they are not accurate enough on Ammonia, Nitrite or Nitrate and go out of date well within there stated range.
The gulping of air is due to the fact that Nitrite inhibits the ability for the blood to carry oxygen, therefore a symptom and not a cause issue, yes you need to heavily airate whilst there is nitrite present but this is not what made the rays sick, it was how you noticed there was something wrong.
If you see them gasp or if the water goes cloudy then that is your sign to do a water change of 25% to 50% ASAP.
The info from Johnno, David and bcfd is all you need to get this sorted