Using DC current to operate a fish feeder that takes 2 AA batteries
Judging from the picture that you posted:



You have an unregulated DC adapter. See this link for some more details. 

In simple terms, the amount of voltage that your adapter puts out goes down as the amount of current draw goes up. In your case, the adapter is rated for 3 VDC at 1000 mA, so it will put out:


3 V when you draw 1000 mA
&lt; 3 V when you draw more than 1000 mA

  3 V when you draw less than 1000 mA (as you've measured, 6.46 V at 0 mA)



Since I don't know anything about your fish feeder, I don't know how much current it'll take or how much voltage is safe to put on it. It might work. It might not. If you can get your hands on a regulated 3 V adapter, you and your fish would be much happier.