I turn on my heat or ac and a very fishy smell comes out ,what could this be ?
There could be a dead rodent in the passages of the HVAC system, and–if so–that could be a significant health hazard. I suggest that you don a respirator mask and exam gloves, and take a look inside the system after the cabin air filter is removed. If you are not able to do this yourself, take it to a mechanic for examination and clean-out.
Even if there is no rodent intrusion, you might have some sort of fungus growing in there. In that case, repeated sprayings of Lysol spray–while the HVAV blower is running–would be a good idea. There is also a spray product called Frigi-Fresh that you can buy at auto parts stores.
Fungus growing is quite common. One person I know has that horrible sour sweaty smell coming out of his air vents when he puts the A/C in the vent position. It disappears gradually on the A/C cooling cycle when refrigerated outside air comes in. Any A/C shop can clean it out. I use a spray into the air intake out side to kill any fungus. This is done with the system operating and all car doors open.