I made some jalepeno poppers, from scratch, grew them in the garden, fried them on the stove in oil.
I learned two things.
1. Jalepeno peppers can be VERY hot tasting, much more so than say another batch of normal jalepeno peppers bought from the store. The garden was great, under watered and it was a hot summer--they were hotter than anything I've ever grown before. Totally inedible (and I'm not wimpy on hot tastes).
2. Donuts fried in that oil will also be HOT tasting, and they will also be too hot-spicy to eat.
If frying bread or anything (like a donut) in that same oil had taken out the spicy hotness from the peppers, then the donuts in the second batch would not have been spicy--but they were.
I wouldn't use oil over from fish/seafood, unless I was making more fish/seafood.