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Hi RES lovers~ I have 2 RES (female: 3 inch, male: 2.6 inch). They've been together since they were hatchlings. I've noticed that they tend to sniff each other's rear ends and stick their ends under the other one's belly, especially the male. They're about 2.5 years old, so they haven't reached sexual yet. There haven't been any signs of biting or fighting, so I don't know whether I should be worried. btw~ My tank is 30G long with heater, proper filtration, and a basking dock with UVA/UVB lights. I appreciate all advises.

Doesn't sound like aggression, but mating to me.
If you see them fanning their claws at each other or biting (obviously) that's a sign of aggression.

just to let you know you cant tell sex until 4", so both could be male or female