Something about the spray bar is attracting them, in nature they will often skim the top of the water to catch insects that hover over the water.

When I feed my frogs nightcrawlers I do not have to place them in the water, just dangle them a few inches about the surface of the water. They will leap up out of the water for food and catch it.

They will go after fingers and I once was bit on the nose when I was trying to adjust the thermometer they insist on dislodging.

Harmless but rather funny.