Is it true that if you have lots of turtles in the same tank the big turtles will kill the smaller ones?

Yes, much larger turtles can hurt and even kill smaller turtles.Turtles need lots of space so if you have an over crowded tank you need to separate them. Not necessarily. If your turtles have an appropriate amount of space they will happily exist with new additions to their group. However, if you are keeping turtles in an aquarium and in very unnatural surroundings, the stress alone of their situation often results in cannibalism.

If the turtles are of the same species, it is extremely rare for larger turtles to kill smaller ones. This is an inherent instinct that helps to ensure the survival of the species. I have, in my 30+ years of breeding turtles and tortoises, found that larger turtles may attack smaller turtles of a different species, either in combat or for food. For this reason, you should never mix turtle species in a habitat without paying careful attention to the difference in their sizes.