Too much of a good thing, perhaps? I've been using these to replace my cats Temptations. My Angelo, who is Diabetic and my Blackberry, who isn't . I have been feeding them their regular pate and some tuna and they get a few packets of these a night. I have noticed some throwing up(usually in the wee hours) from both and Blackberry had diarrhea this morning. Angelo has been off insulin for about 2 weeks.
I would be careful with too much tuna.
I read our can the other day and saw there is soy in there! Which, I haven't really dug into yet but from a quick glance it appears that soy can mess with the thyroid which can mess with blood sugar. I had no idea there was anything but tuna and water cause thats how it appears on the front of the can (solid white tuna in water).
Also aside from the mercury risk with too much tuna... our guys ...if they're getting special treats like tuna/chicken/sardines will eat less of their regular food which they really need the extra balanced nutrients from that the treats don't have. And I think I was not noticing they were consuming less of their canned food while they waited for the good stuff to be brought out lol
Personally, I have decided to not give them tuna at all anymore because of that soy, we used to give them the juice and a small amount whenever we'd have tuna, which is already not that frequent but from what I've read sardines, boneless skinless in water, are a lot healthier in general, b12 and omega3s and because they're a much smaller fish they're not the mercury risk bigger fish like tuna are. And the "Season" brand has skinless boneless in water, and the ingredients really are what the box says on the front, nothing but sardines and water.
How frequently are they getting the tuna and how much is in those packets? Cause it might be too large a percentage of their daily food intake that isn't nutritionally balanced, I'm not sure.
Also, in my reading trying to decide if tuna vs sardines were safe, etc... I came across something that tuna can cause...just googled the name, seems like it's rare but worth reading up on if you give a lot of tuna, its called "steatitis" and apparently puking can be a symptom but puking is probably a symptom of everything lol.
https://www.cathealth.com/cat-health/digestive-system/2387-steatitis-yellow-fat-disease-in-cats