I have added everything from frogs to salamanders and turtles to my koi pond. Recently the bullfrogs have been croaking and I saw some slime coated eggs I assumed were frog eggs. I noticed them about 4 days ago and placed some in a container to observe. This morning they hatched into what look like black worms that wiggle up to get air and then return to the bottom. Can anyone identify these things?? I know they just hatched but if they are tadpoles, I kind of thought their shape would look like tadpole, can anyone help identify them?

Thank you for any help
picture below