We eat a lot of tinned tuna in our house. One of my 3 kids has a tuna wrap with sweetcorn for packed lunch every day (about half a small tin of tuna), and will probably eat it in a pasta sauce or on a baked potato at some point in the week as well. Same ds will also have one meal that is c.5 fish fingers. He doesn't like meat.
Recently someone told me that it's too much because of mercury levels. It never occurred to me to worry about mercury levels as I was just glad that he likes tuna, when he won't eat chicken, ham, cheese, or other sarnies that most other children like. In the winter I tend to make a big minestrone that I can give him as an alternative in a thermos on some days, which he loves, but is the tinned tuna thing really serious? I read this article which seems to say you can eat as much as you want.
www.nhs.uk/Livewell/Goodfood/Pages/fish-shellfish.aspx#much2