When my fish start sitting, it's usually an early indication of something nasty

But this is the time to start tackling whatever is bothering it (water quality issues, parasites etc) - fish usually make it if you treat it as this stage.
Do you have test kits to tell us what the water parameters are? Also try and tell us as much information asked in the red box above. E.g. look at the fish closely and describe anything abnormal? Are its dorsal/top fins clamped?