Arowana Gasping

Cleaned and changed the water in the aquarium because some reddish algae were growing on the surface of the aquarium. The next day, I noticed that our arowana was gasping for air - kept opening its mouth very frequently and later remained at the bottom of the aquarium ! Something was wrong with the water. Quickly changed some of it, add more aeration and put 'yellow Japanese fish medicine' as precaution. It slowly swam to the upper level of the water and remained there, not moving much and still gasping. The next day, it did not open its mouth that big and no sign of gasping. It remained at one corner of the aquarium, near the water surface. Swam slowly every now and then. The following day...it went back to its old self. Up and going again. 

Prescription : We could only try our best and hope that everything would be fine.