Welcome to MFK, some things you should know about a silver arowana.
They should be raised solitary until about 5 to 6" long and then with only very select non aggressive tank mates up to about 10".
When kept with cichlids the arowanas length should be twice that of cichlid.
Arowana and gar are not recommend as tank mates.
They grow very quickly 1 to 1.5" per month and will be 24" long within 18 months. This means that if the aro survives it will outgrow the 40g in 4~6 months. A year from now you will not have a tank that is proper size for keeping an arowana.
