Too much tuna?

When it comes to fish, the biggest concern is usually mercury, and the rule of thumb is the higher on the food chain they are, the less and less frequent you should eat it, due to the compounding effect of mercury ingestion by everything below the fish in question on the food chain.

That said, the mercury content of canned tuna is usually pretty minimal due to the type and age usually used for canned. Different species of tuna have different levels of mercury, so it might be a good idea to see what kind you get (white vs albacore vs yellowfin) and try to stick with the kind with the least amount of mercury.

Any of the small fish that you eat whole, such as sardines and mackerel, don't contain much, if any mercury at all, because they eat things like plankton. The same goes with salmon.

Here's a list of the relative mercury levels in fish - http://www.nrdc.org/health/effects/mercury/guide.asp

The other concern with canned foods in general is the BPA content in the lining used by many canned good manufacturers (most cans are lined with a thin coat of plastic to keep the integrity of the metal can and keep it from leeching into the food, but the BPA can also leech into it). There's some evidence that it can cause a number of pretty nasty effects on the body (endocrine disruption, neurological damage, thyroid dysfunction, cancers, and a few others), but the other concern there, too, is that BPA substitutes haven't been studied at all yet (they're simply too new for the most part) and their effects (particularly long term) are very much unknown. So, in that regard, you'll need to use your own judgement on it.

If you're concerned about it, you could consider canning on your own. There are a couple of glass canning jar manufacturers out there, which bypasses the whole thing altogether (using glass means no need for liners, and glass doesn't leech). If you live in a dorm, you could make trips to your parents' house (or a friend's or whatever) every so often and can up a big batch of stuff to take back. That has the added bonus of being able to control what goes into the food and getting higher quality food, among other things.

Here's some more info on the studied health effects - http://en.wikipedia.org/wiki/Bisphenol_A#Health_effects

As for the foods themselves, aside from the above, there's nothing really inherently bad about them, even from eating them a lot. Just make sure you're getting plenty of fats and enough calories in general to fuel your day. The body doesn't do well living on protein alone.