I use a cup in the tank, and a rigid PVC tubing, and a turkey baster.
thaw and mysis, add some SW, use turkey baster to suck in some mysis, put the rigid [clear] PVC in tank with the end in the cup and other end outside the tank. then with the baster inject the mysis into the tube. the mysis moving down will attract seahorses, they follow it into the cup where it will stay for them to eat, and you can easily remove the uneaten portion.
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