Hi there,
sadly its not possible to be prevented if you put the fish in a tank. As normally a tank is positioned quite above the ground, fish will be tempted to glance out of the tank looking down.
Putting things that float in your tank "might" help? the idea behind this: fish's attention is supposed to be drawn into that material instead..of course this is arguable as some ppl might find it useless and ineffective...
Its a different story if you keep em in a pond as far as i know, heard tht the chance of having drop eyes could be drastically reduced...
Correct me please if i made an inaccurate comment
Leon