Shell rot or shell shedding?

[suyuen]

Hi! I'm an owner of a red eared slider for 33 years. Recently i notice my turtle's shell having a giant crack but there's no way for my turtle to fall or crack itself in its enclosure. I also can't tell if it's can actual crack or just his shell marks being deeper than usual. There's also no way for other animals or creatures to come close to it as it has netting around its enclosure.

Apart from the crack I see new black colored mini dents on its shell which I've never seen in my 33 years of it being with us.

I decided to Google and found it can be either shell rot or natural shell shedding. However upon turning him around I notice the bottom that used to be mostly yellow is now mostly black. Do you know if this is normal?

Thanks in advance!!

 

[ZEROPILOT]

I'm not familiar with issues with water turtles.
I assume he has clean water. I see what looks like algea on his carapace. But I see no fungus.
Could his lighting be drying him out or burning him?

 

[suyuen]

He lives in an enclosure outdoors in tropical Malaysia. Parts of his enclosure gets direct sunlight at certain times of the day. I do see him climbing onto his island to bask in the direct sun sometimes.