Corn Snake refusing to eat mice

[Tilted]

Hello,

This is my first post here but looking on some feeding advice for my pet corn snake. It's always been a struggle to get my snake to eat his pinkies, even when brained. The only way I can get him to eat them is if I soak them in tuna juice, but I am worried that he might be getting too much mercury from the tuna.

I have tried feeding him a sliver of salmon and he loved it. I also started feeding him whole sardines, as I feel this will give him a more whole nutritional profile. He is still too small to eat live animals and I am wondering if its okay to keep feeding him sardines or if the mercury in the tuna juice is negligible? Is there any other substitutes or methods of preparing his meals you'd recommend?