Why is fluid leaking out of the expansion tank on your 96 Volvo 850 GLT?

if u mean coolant level... check for an air lock ( interior heater on full and hot rev engine top up if nessesary) could be thermostat or headgasket too

It is most likely due to a hairline crack in the outlet pipe of the expansion tank. Common fault, ask any Volvo dealer. As the vehicles age, this plastic becomes hard & brittle, so small cracks are almost inevitable. Also check your coolant cap for the same sort of cracking.