Help identifying mystery swimmer?

Unfortunately, I don't have a picture of one of these guys, as they're constantly moving and the water is murky, but I'll try my best to describe:

So, I keep a little pond tub full of water outback because the local frogs enjoy spawning in practically anything and I enjoy seeing all the little tadpoles and tiny juvenile frogs that come as a result. The first "friends" they shared this new home with (as I used to have an abandoned pool that played home to the local critters, but we got rid of it) were a trio of diving beetles. I shrugged, no idea how they got in there, but understanding it was part of their ecosystem.

Now the tadpole armada has some new "friends" occupying their space and I have no clue what they are. They're slim and limber, maybe half an inch to an inch long. They're a mottled brown color with two black stripes running down each side. They have a head separate from the body and breathe through a tube on their bottom. The only time they stop for a second is when inspecting some food or breathing. I know they're not water scorpions; I thought they might be Dragonfly nymphs, but I've yet to find a picture of those that look like them and dragonfly nymphs seems to be shorter and thicker than these guys.

I live in North Central Florida.