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my small turtle, dana, seems to be very aggressive lately. He/she (i'm thinking he) will chase my bigger turtle, mercedes, around their tank. sometimes dana will even try to bite mercedes on the back of the neck or her shell. are there any certain reasons for this?

mine always chase eachother around. How big is your tank? Dana might want Mercedes out of there, mine did that, until i upgraded and they seem fine. Use the aquarium rule (10 gallons for every inch....) and see if its big enough. If it continues, you might have to seperate. If there any fluttering with their claws near eachother? Mine are (1 male, 1 unknown yet) and its also a sign of marking territory, ask someone else, i'm no expert

E.T

i don't really know what the fluttering of the claws are and i'm still not sure about the sex. right now they are in the biggest tank i can afford but i hope to upgrade soon. i think one is a male and one is a female, but i'm not sure.

Please post pictures of the cloaca and the nails of the two turtles. We will be able to help you determine their sex

If this continues and possible injury might occur, seperate them no matter the sex.

ellman605 wrote:Use the aquarium rule (10 gallons for every inch....)
E.T

This rule is beneficial, but it is not completely necessary. Following this rule in some cases will be ridiculous. Turtles can thrive in gallons that are a bit smaller, and each new turtle should not be a result of adding how many inches and gallons to the tank.
I generally go by this rule for a single specimen - Gallon size canbe .5inches smaller than the turtle
E.G. - a 4 inch turtle can thrive in a 35 gallon tank.
a 9 inch turtle can thrive in a 85 gallon tank.
But it is always important that the biggest enclosure you can possible get is better.

An additional turtle, no matter the size, should extend 30-40 gallon in the enclosure.

How big are your turtles and how big is the tank? Is there a place that the one can go to for privacy and to be away from the other? Are there any objects in the tank to break up their sight lines so they don't have to look at each other all the time?