arowana health issues

I had a fish arrive belly up floating but still swimming around. What could cause this? How would I treat it next time?

Silver arowana around 14cms, is stationary in front of aerator pump and gasping for air. Mine is a 75 gallon tank, recently(almost a month back) the nitrogen cycle crashed and there was ammonia spike , so I have been doing continous 30 percent water changes in alternate days and am maintaining...