^ Sounds like some species of soft-shell turtle. There are a couple different species living in India. All of them get quite large, especially the females. They also do poorly with fish as adults because well, they eat them! Letting the turtle go some place safe is a great idea. The turtle will thank you!
. one very important note: release it afterdark, or make sure there are no crows/ravens or eagles if you release it during the day time. they will kill and eat it if they can.
