I want to start buying those big tuna cans to save on costs, but I know that after one day at the fridge opened tuna becomes stale. It loses its tangy flavor and gets a rubbery texture. Is there a way to prevent that?
At the very least, transfer to a food storage container (smallest possible) and cover...for more than a day or two, Ziploc bag, with as much air removed as possible (use water displacement method)...or vacuum seal.
It works best if you use the tuna first and make a salad which you can store, but what I do and it works very well is to find a bowl that is almost exactly the right size for the amount of tuna, but still large enough for there to be just a touch less tuna than the bowl can hold.
Then cover the tuna with plastic wrap and push the wrap down onto the tuna so there is no or as little air as possible trapped under the plastic. Don't stretch the plastic over the top of the bowl, but rather press it down into the bowl over the tuna. I have successfully kept tuna salad very fresh this way, and when using certain seasonings, a day of marriage in the fridge can make it nicer than freshly mixed tuna salad.