Softshell, leatherback, archelon 
For small ones some species of musk turtle is the way to go. Not a stinkpot necessarily as other species fit the bill also. Problem is while they're aquatic you might not want their water as deep as you would for fish so depending on the planned setup the mud or musk turts might not be the best option.

For small ones some species of musk turtle is the way to go. Not a stinkpot necessarily as other species fit the bill also. Problem is while they're aquatic you might not want their water as deep as you would for fish so depending on the planned setup the mud or musk turts might not be the best option.