ethoxyquin is in a lot of fish foods...people foods too for that matter actually.
Many will tell you not to worry about it becuase it is a fairly low amount in most brands. But the risk is still there. many people never have trouble with feeding these foods though and their fish live long healthy lives
I personally try to stay away from ethoxyquin, but i still on occasion feed a bit of tetra or omega one flakes that do contain it. I just try to think about it a bit, but if they get some of it i don't worry. Its just something to think about (check other pet foods, they often contain it as well).