Will copper pennies harm the fish in a small fountain or aquarium? I think the fountain is 8 gallons.

Copper will not harm fish.

and I don't think there's all that much copper in "copper pennies" anymore -- is there?

from livescience.com:

"In 1962, the cent's tin content, which was quite small, was removed. That made the metal composition of the cent 95 percent copper and 5 percent zinc.

The alloy remained 95 percent copper and 5 percent zinc until 1982, when the composition was changed to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). Cents of both compositions appeared in that year."

Apparently we need to change our thinking to "zinc pennies"

I have read in fish tank forums that they are used to kill snails and inverts. They can also kill scaleless fish.

I have read the the true older copper pennies are good to use in bird baths and fountains to help control algae. Has anyone else heard this?

They put copper in anti-fouling paint for boats, and that does inhibit marine plant growth. In 'olden times' they used to sheath the bottoms of sailing ships in sheets of copper for that purpose and to protect wooden hulls from barnacles and ship worms. Copper is also a component in algaecide used in swimming pools to keep algae growth down.

Here's an article on it:
http://copperantifouling.com/c...

Scroll down to point #5 on Environmental impacts if you don't have the patience to read the whole thing.

Here's an older thread from 2013 where this same question was asked here:
The thread "Pennies in the birdbath..." in Ask a Question forum