How to prevent unwanted taste transfer?
Aromatics are at least half of taste. Its like when you go to a restaurant and have to wash your clothes when you get home because they smell just like what you ate. Potatoes don't have any particular dominating flavor, they generally inherit whatever flavor they are exposed to; chicken is much the same way. Tuna on the other hand has a very pungent aroma. I wouldn't be surprised to see the fries take on the tuna aroma and "taste" a little fishy. Try temporarily covering your fries while you cook other things. That should help.