i lost a 10inch silver and a 4 inch super red when, while doing a water change, my son turned the hot water on full and i had a 5 degree increase in temp in probably 8-10 minutes. lost everything (including the silvers) with the exception of 3 full grown oscars and a 8inch blue dragon flower horn
seems like your water parameters is not that bad, arowanas are pretty hardy fishes, it think there is an external factor behind this, something might have startled it or might have attracted it to jump, sorry for the lost...