I smoked bluefish and stripers in a smoker about 10 years ago. Even with a powerwash, I could not remove the strong smell of fish, and when I fired it up, the smell intensified.
I had to buy another smoker for meat and use that smoker only for fish.
Oil from the fish will leave a strong odor in your cooker.
Will it affect other things you cook? I don't know and did not want to find out, but the smell was terrible.
I would suggest to get a cheap smoker to use for fish only. When you smoke fish it is not usually in large quantities so a small smoker like a "Big Chief" or something similar would work fine as a cold smoker or as a hot smoker.
Maybe it has to do with the type of fish you smoke.
Humphrey's DownEast Beast W/BBQ Guru