Sorry to hear that; too late to help, I know, but the next time you notice a potential problem you should test the water right away, rather than the following day. If you get a reading like that you should run to the hose to do a water change, not to the store for a solution.
When you have bad water...get rid of it and replace it with good water immediately. Then track down the source of the original problem and correct it so the issue doesn't recur. In this case we don't know where the ammonia spike came from but maybe if you retrace all your steps the source might become apparent. High nitrates in a tank that was presumably filled with fresh water only two weeks ago would seem to indicate that the cycle was preserved by utilizing the mature media. It also is a hint that you need to change more water more frequently. But the ammonia spike in a cycled tank is puzzling. Dead fish? Extreme overfeeding? Did you add a bunch of other fish at the same time as the Arowana? Any of those things might have temporarily overwhelmed the capacity of the biofilter to completely process a spike.
If you already have a bottle of something to detox/lock the ammonia right there on the shelf, that might be the fastest and most immediate bandaid to apply; maybe get some product and keep it on hand for future problems. But keep the hose handy!