I think size might be the reason, fish needs to think of it as a rival. On adding it to the tank fish would know it's not another fish.I have found when used outside the tank, they will chase them back and forth like they would a finger or anything else you may use. When dropped in the tank they usually nudge them a few times, but seem to loose interest fast. Here is what I have, maybe there are some more life like toys that work better?
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I have seen people make their own toy using cardboard and a print out of flowerhorn picture.