How big is it, I have kept Baby Spinys up to around 5-6 inches with other Turtles with no issues, but after that they start getting aggressive. As babies, they are really docile and more curious than anything, I have one right now in my 90 Gal with a RES and a Map and haven't had any issues. But beware in a 2-3 years that will change. fish are also a tricky matter, some go after them while others do not, my current one refuses to go after fish because he is well feed. That is also a issue with Softshells, watch how much you feed them, Softies tend to over eat if you let them