I ate Tuna in a pouch for a few days.  It seemed like a very paleo and
convenient snack to have during the day.  After a day, I didn't feel as
perky as usual.  After two days, I felt pretty rough.  After three days, I
couldn't focus, I couldn't go to the gym, I slept an extra two hours above
normal, and my whole body felt heavy.  So, as an experiment, I stopped
eating the tuna and I felt fine within a day or so.  Within two days, I
felt great :-)

It's only anecdotal, make of it what you will.  I put it down to the
mercury levels in tuna, but that's just an uneducated guess.

FYI: I now eat salmon, haddock, or trout on a daily basis and it makes me
feel more focused, energetic, clear, and positive.  Quite the opposite to
my experience with tuna.

Kat.