You want turtles that are about the same size, and will grow to about the same size. A 4" slider might be bullied by a 6" slider, for example. Also, the size of your tank often defines agression in aquatic turtles. Two females that get along fine in a 1000 gallon pond will rip each other to shreds in a 100 gallon tank.

Avoid mixing muds or musks or snappers with anything else. Also, male painted turtles are very agressive towards females.

Katrina
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1.2 Eastern Muds - Fred, Ethel, Edith
0.1 Iguana - Tiffel
Foster turtles: More than I'd like the husband to know about.