For a tank that small, with only 2 fish, I wouldn't give more than 1/3 of a frozen cube per day (I am assuming it is a cube?)
What I do is get a water bottle cap (with a nice flat top), and put one cube in it (the cap is upside down, obviously), then take a dropper (like an old medicine dropper with a squeeze bulb on the end) and add tank water to the cap with the cube in it (not all the way up to the top, leave a little space for thawing unless you want stinky overflow). Wait 5-10 minutes for the cube to thaw, then using the same dropper, suck up some of the food and squirt it into the tank. Wait until the fish eat it all before adding more.
When you are done (like I said, about 1/3 of a cube per day), put the cap with the remaining food in the freezer for tomorrow. The next day, just add a little tank water on top of the frozen food to thaw it. Repeat as necessary -- you get the idea.
For a small (or nano) tank, you want to keep the excess nutrients out of your tank or you can have problems with nitrates, etc.
