Have the cabin air filter changed.
There are a number of reasons why air coming up from air vents would smell like dead fish. The air ducts may need cleaning. The air coil may need replacing. It could even be an electrical problem with the unit.
sounds like your air conditioner compressor is bad.
Perhaps a dead mouse.
Most of the time it is caused by a hole in the heater core, do you smell a sweet smell? The sweet smell would be the antifreeze.
heater core, tough job.take it to a qualified auto technician
Any smell from the car vents is unrelated to the AC system unless it is an antiseptic smell. Generally, the smells are environmental from your engine compartment, where the fresh air is taken from. If you smell something burning from the vents, then something is heating up to much under your hood. Check your compressor for a worn clutch, or oil leaking onto the exhaust manifold.
Return air vents.........
More than likely the heater core is leaking.
Check the fan motor itself. It may be shorting enough to cause it to run hot. the smell may by from the motor itself.
Smell may be (heated) mouse dropping and urine. We found a mouse nest in the (and partially made from the) air filter. Open the air filter and check, mouse was still hiding down the air hose. This search was done in an attempt to find that sour smell.
the heater core may be plugged up