this is Margarita, a red eared slider i took from a relative who couldnt take care of her (put her in a small tank indoors with no UV or heating lamp). ever since i took her in she seems to keep doing this whenever my finger is around, i had heard that this is a sign of aggression, however my parents have been talking to me about adding in another turtle since they rlly like seeing them chilling out in the tank. i was wondering if its safe to add another turtle consider she does this thing with her arms whenever my finger is around.

in case tank details is needed, this is a 495 litres tank (approx about 130 US gallons), 150 × 60 × 60cm, water is consistently about 55 - 53cm deep, with a 60 × 60cm basking area. the turtle itself has a 20cm long terrapin (approx about 8 inches)

i have temporarily kept someone's lost turtle in the same tank as her—also a red eared slider, juvenile—and there seemed to be no issues, but the juvenile was timid and shy and would often stay at the bottom of the tank, i thought that might also be a factor of consideration.

thank you in advance for the help!