The form of zinc used in the studies on strength and testosterone was Zinc monomethionine aspartate. The zinc form that you're currently taking and the zinc form used in the studies are both chelated forms of zinc, but the studies also used the chelated magnesium aspartate as well as B6. Studies have had mixed results, including a German study in 2006 that showed no statistically significant increase in serum testosterone levels. The amount of zinc monomethionine aspartate used was relatively high at around 20-30 mg per day, with around 400-500 mg of magnesium.
If you want to duplicate the results of any of the successful studies, then I would suggest taking the same form and dosages used.
As for the mercury in tuna fish, unless you lack the sufficient enzymes or have pre-existing liver issues, consuming a can of tuna fish per day should not pose any threat to your health, as a healthy person should be able to eliminate this amount naturally. BTW, there is naturally occurring lead in many mineral and herbal products, so if you are concerned about heavy metals, you may want to also take that into consideration when taking high amounts or potencies of herbs and minerals. Again, you shouldn't be too concerned as a cup of carrots has been shown to contain 12 mcg of lead and any product sold in California can not have more than .5 mcg of lead without a warning label (or there not supposed to, although many still do, until Prop 65 attorneys sue them).
If you're really concerned about heavy metals, here is a product that I hold in high regard- Invalid Link Removed
The clinical research on the ingredients used were pretty impressive as well.
Most chlorella and cilantro heavy metal detox products actually contain trace amounts of lead (how ironic when you may be trying to perform a lead detox) and many zeolite heavy metal detox products will contain trace amounts of aluminum. The product mentioned above does not have detectable levels of any of the metals mentioned, which I found quite remarkable, since most of the vendors selling chlorella, will usually have small or trace levels of lead.
Children and a small amount of the population are at a higher risk of heavy metal toxicity from naturally occurring amounts in food, supplements and municipal drinking water, so I wouldn't stop eating tuna fish or be too concerned.