First of all, if they do not have a lid, then eventually a bird or a snake will gobble them up! It could be that they are afraid. Also, if you have three or more in a tiny container, then you need a bigger container. Also, is the container a color???
If the container is a color, then thats your problem right there. If it is clear then it must be something else, but if they cannot see the outside, then they dont have dreams or imagination, and they wont be able to see anything except this one color. They minds will not grow and if the minds do not grow, then that results and them not growing!
Other than that, I cant really think of what other possible problem could be with your turtles. Do not feed guppies or any fish to a baby unless it is a juvenile!
Slave to a bearded dragon, two turtles ,two dogs, two horses