It's good to talk things out to help develop ideas. What if the fish become invasive, for example? It invariably happens, see all the times we accidentally introduced X species (usually rat) to non-native lands, then intentionally introduced Y species (bird or cat most often) to deal with X, and instead X and Y are both flourishing, devastating the local ecosystem.

Biological solutions are almost always dangerous and difficult to control like that, unfortunately.

Magnets don't reproduce, so generally there won't be magnet-outbreaks, you know?