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There is a little device, sort of like the garlic press, made especially for draining canned tuna. It fits right over the opening, push handle down and fully drained to your liking. Most grocery stores, around my location, have them down the canned tuna aisle. If not, I'm sure you can locate one on Amazon.
By cate0330 
I have struggled with an easy way to drain the liquid off of a can of tuna. Usually I just use the lid of the tin to press the liquid out, but it rarely gets it dry enough for my taste. I searched everywhere to find something more efficient, but it was right in my utensil drawer all along! Its a metal lemon squeezer! I just put the contents of the can in the inside part of the squeezer, press the top part of the handle down and squeeze until it's as dry as I want it. You may have to stop to push some of the tuna back in that comes out around the edges, but it works great.
By B 
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Here are questions related to Draining Canned Tuna.
What is the best way to drain a small can of tuna?
By Judy = Oklahoma 10/08/2007
My can opener lifts the top off instead of cutting it out, so I can't use the lid to drain the tuna. I have one of the sieve-things but can't ever find it when I need it! But I DO have a drinking glass that has a bottom that is the same size as the can opening. I put the glass in the can, tip the can sideways over the sink & push the glass down until it drains.