Im afraid to tell you but your fish is probably not actually freshwater but a brackish/marine dwelling species of the Batrachus genus which will not live long if kept in freshwater.
Fish such as these only need feeding a large meal every three to four days.
It could be a Notesthes robusta or Bullrout. These are native to Australia and can be found in freshwater creeks. I've never heard of them being for sale commercially though.
Although Notesthes robusta is occasionally found in the wild in freshwater (as well as brackish and marine), it does considerably better in captivity in brackish water and will feed much more readily.
A store near me has one right now and I keep having to stop myself....