Sick tadpole with red mark on belly

[Grace20]

Hello all, I'm new here and new to posting!
I have a tadpole that has become a little bloated with a red mark on its belly. I looked up diseases and it might be red leg? I have no idea. I have placed the tadpole in a separate pond from the other tadpoles in case it is infectious and i am keeping an eye on it.
Does any one know what I should do?

Not sure what frog he will be as the tadpoles were rescue from being put down the drain, but as moterbike frogs are the most common in my area i think that's a safe bet.