After cooking fish, it always smells for several days even with the window open. Is there any way to get rid off the smell faster?
Prevention is the best way:
- have a window open (as you already suggested);
- properly install a good kitchen hood - not too high away from the cooking area, with a high air flow;
- if possible, use a cover on the cooking pot, tray...; If you cook it in the over, you may even try wrapping it in aluminum foil;
The hood should be started before the smell starts spreading, and be stopped "long" after finishing cooking.
Post mortem, you may be successful by using an air ionizer, it might be able to break the smelly molecules floating in the air.
A passive apporach would be to create "strong" ventilation in the smelly area. If the weather is windy, just leave a few windows open. You may use a large ventilator, if the re is no wind.
Be careful NOT to spread the stink in all your apartment, depending on the direction of the wind.
Since thoroughly airing out your kitchen doesn't help I would suspect that there are still some (possibly very small) fish parts around.
Get rid of them: take out the waste, make sure that pan/pot and plates are clean (and not just waiting in the dishwasher). Clean all surfaces that might have come into contact with the fish (or with fat that the fish was fried in / condensation from the steam from cooking it). Also wash or throw out whatever you used for cleaning.