Barramundi sick, help!

[Svampus]

i've had my barramundi for maybe 2 years now, he has grown from around 40 to 60cm+. Been eating mostly different kind of fish and shrimps, temp always around 25-27C. 100% freshwater. He always comes up to the front glass to beg for food when i get close to the tank, and is always first to eat. But the last couple of days he has been pretty inactive, has not eaten when i feed, and also has his fins along with his body. Not out and "waving" like most times. My RTGG has been a bit aggressive against him some times and he get scratches every now and then, but it has not affected his behavior or eating habits... Right now he has some scratches. All other fish is eating and behaving fine, i really doubt that the water is bad in any way...

But, is it possible that when barramundis grow to a certain size they NEED a bit of salt or maybe even brackish / saltwater to continue living? Please, if someone know or have any ideas / tips of articles or anything write it here! I really don't wanna loose this fish