You should not use a hammer. Get a dull knife like a butterknife. Look at the picture of the clams above. The one on the top right of the picture. The part of the clam directly above the tip of the knife is where you stick the cutting part of the knife, not the point of the knife. Hold the clam with the "hinge" of the clam (the part facing the top of the picture) in your palm (near your thumb) and push the knife blade through the clam by squeezing your fingers. The knife should go right through. Then wiggle the knife back and forth to cut the adductor muscle that holds the clam together.
Freeze the clam then open it all the way and slice off (with a sharp knife) paper thin slices. Feed that to your fish.
Blackworms are great and provide fish the bacteria they need to stay immune but clams would be a better food.
Sometimes a copperband will not recognize a whole clam as food and they don't have any teeth so they can't rip off pieces.
Good luck, if all else fails, use a hammer.