I'd go with Snakeguy, and his mata mata. If you are wondering, its right at the bottom of the tank. The video you saw seems pretty flawed to me, a slider in a fully aquatic set-up gets cranky and will end up squishing your fish. Softshells are ambush predators with an attitude for destruction, when I was somewhat younger, my entire tank of silver dollars got ripped apart by two of the apparently innocent turtles.
As for the mata mata, it doesn't care to bite you, or your fish, just as along as your fish aren't smaller than its extended mouth and you give it a good supply of food. A bad part is that it is pretty inactive. Don't have experience with them..so this is all I can help with.
