First, you should never, never, never, place frozen -anything- in your tank! Whether it's a cube or the shrimp, or whatever.
Let them defrost in a cup, bowl, whatever, before you put them into your tank.
Don't want to give your fish a brain-freeze do you
I feed my snowflake eel two or three (total) shrimp (poppers) a week. I typically break them into two peices, to make them smaller for him to eat easier.
I also feed mysid shrimp (one defrosted cube) on an off-day.
Lastly, once a week I get either a small goldfish or a couple of larger guppies for him to try
to catch and eat