I always deep fry my fish in Canola oil which cooks the fish really good and also makes it taste very good as well.
I've also used peanut oil to deep fry fish in as well and it cooked the fish just fine and gave the fish a good taste but now that I tried Canola Oil I prefer to deep fry all my fish in Canola Oil.
Canola Oil is pretty cheap and a great oil for frying fish as well as chicken, potatoes and much more in.
Canola Oil is also healthier for you than many other oils are but peanut oil is also pretty healthy for you when it comes to cooking oils.
Most places do use Canola oil to deep fry fish but I've known some use peanut oil.
You can also use regular vegetable oil for cooking fish in as well but Canola oil in my opinion is the best cooking oil for deep frying fish in.