We need much more info. What has it been feeding on? How large is the
fish and tank? It sounds to me like she may have
been feeding live despite claiming not to normally feed that. Feeding
live FW fish to a SW fish is going to kill it after
several weeks to a few months. It can be hard to wean off live but that
is no excuse...take a toothpick and poke a feeder
and also poke a piece of shrimp, cod, scallop...and wiggle at the
surface. After doing this for a few days try just with the
dead food...fresh or thawed...never feed frozen to a fish. Its hard for
them to swallow and just cruel. Live feeders
will cause the fish to deplete its liver for energy which assists in
swimming by providing bounancy. That may explain the
upside down swimming. Lions and other large predatory fish can't be kept
in hyposalinity because it makes their
kidneys very inefficent. The fish will have a toxic buildup of waste
products in its bloodstream therefore killing the fish
or causing kidney failure/death (sort of the same thing).

JK