My Son Caught Three Red Eared Sliders And I Put Them In One Of My Man Made Fish Ponds. In The Morning One Was Lying On The Bottom Upside Down And Dead. What Could Have Happened?

Why did my turtle die?I thought the turtle drown, because my husband removed a rock that I had for the turtles to climb up on. There was a large plant in the water that I thought they could rest on if they needed to. I had this happen one other time. The turtle had only three legs, but there were rocks in the pond that he could climb out on. Luckily I noticed that one was upside down on the bottom sooner and was able to revive it. I then transferred it to a natural pond. .