You floated the Arowana for 3 hours?...
Floating for 20 minutes would have given the same results. Since the Arowana was so small and delicate you should have floated him for 20 minutes and then drip acclimated it until you quadrupled the water in the bag which would probably take 1-2 hours. Thats always the best type of acclimation because it slowly introduces your water chemistry, not just the temperature, to the new fish. So if you ever get an expensive fish, drip acclimation is the way to go to increase its chance of survival.
Floating for 20 minutes would have given the same results. Since the Arowana was so small and delicate you should have floated him for 20 minutes and then drip acclimated it until you quadrupled the water in the bag which would probably take 1-2 hours. Thats always the best type of acclimation because it slowly introduces your water chemistry, not just the temperature, to the new fish. So if you ever get an expensive fish, drip acclimation is the way to go to increase its chance of survival.
