Hydrogen peroxide used 1 tablespoon per gallon will add oxygen to your livewells and save fish in high temps when water doesn't hold much O2.

Basically, hydrogen peroxide is just water with an extra oxygen (H2O2). It will not replace the slime coat, but most fish die in your livewell from suffication. The slime coat comes in later when it can prevent disease.

I'm not knocking the products out right now to add to your livewell, but a cheap bottle of hydrogen peroxide can be used in a pinch.