I am looking for a bit of information on the
ultrasonic/electromagnetic pest repeller devices.  Desired answer
shall include:

1. Review of ultrasonic and electromagnetic (EMF) pest repellant
devices -- most interested to know whether they work, and what the
dis/advantages of each are. A link(s) IS sufficient here.  If there is
a consensus that one or a few devices are "the best", I'd like to know
which. An overview article is greatly preferred to a bunch of
amazon.com review links.

2. I need to know whether these devices are safe for fish, and
specifically invertebrates such as snails.

I would urge caution here. The Federal Trade Commission has indicated
that these devices may be ineffective.

http://www.ftc.gov/opa/2001/05/fyi0128.htm

I don't know whether they will harm your snails, but it appears that
the ultrasonic gizmos have little effect on the pests they are
supposed to repel.

Thanks for the comment. I have since read a few consumer reviews of
such devices, and they've been generally negative.  One stated that it
had no apparent effect on the pests in their house, and they actually
observed a mouse walking right under the device, apparently unaffected
by it.

This person went on to state that the best thing they've found for
this purpose is liberal use of Lysol on surfaces where insects or
rodents are usually found.