of the water, which made some difference. I have found that a bit
of emory cloth to clean the corrosion off the prongs is a simpler (and
cheaper) solution than replacing the bulbs. They still get corrosion, I
suspect because the sockets are still corroded and it spreads to the
prongs.


You might try some silcone grease


Could I cause serious problems by not submersing the lights?


The fixture probably relies on being subursed for proper cooling.



Is copper truly toxic to fish?


Yes, however it depends on the chemistry of your water whether the copper
will dissolve into the water.