I know that many fish are capable of hearing ultrasonic frequencies. I don't know about thresholds for tropical fish, but I think goldfish can hear up to around 50kHz, and research at University of Maryland has found that shad can hear up to 180kHz. I personally wouldn't do it. We have an ultrasonic water bath in the lab for disrupting bacterial cells, and you get a headache if you're near it for too long.
They look cool, though. What were you planning on using it for? To get cool mist on top, or for oxygenation? I would probably only use it in a semi-aquatic or terrarium-type setting, where you had a separate pool with the mister. One thing to note is that any salts in the water are misted (mistified?) along with the water, so you may get salt deposits on everything with this unit.
Hope this helps.
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