I have a European pond turtle and a red-eared slider as pets. They had been living together in harmony in an aquarium for about 8 years, but when I moved them both to a larger aquarium a year ago, I started noticing something weird.
Every now and then, the pond turtle (which is about half the size of the red-eared) would climb on the back of the red-eared slider and stay there. The red-eared doesn't seem to like it and tries to remove the pond turtle from its back, but the smaller pond turtle manages to stay there.
Is this normal behavior, or could it be harmful for the red-eared slider? Should they co-exist in the same aquarium?