So first off, I'm a beginner. I got my turtle 3 days ago. He's a red ear slider, about 1.5 inches. Ever since he started going under the basking spot, there's been this weird black outline on his shell. Some info: i dont have a UVB bulb yet, i've been taking him outside in a large plastic container and letting him get natural sunlight (is this ok even though he's very small?). Also, i dont have a filter yet so i've been changing his water daily.
is this shell rot? shedding? too much basking exposure? he's been going in the water a lot too.
i also bought him a turtle bone that he has been eating (cuttlebone - yes, i've taken off the back), will the calcium help?

