Hi! I'm new to the business of owning/caring for turtles. I have a 5-7 year old (I think) male (I think) Red Eared Slider (the turtle is not my turtle per se but I'm living with my father-in-law and I've taken over caring for him mostly); he was in a bad environment before I took things into my own hands. His tank held 3 gallons of water with a useless filter to boot (and consequently filthy water) that lacked a thermometer to check the water or basking dock temp, etc. Mind you, this is not a small turtle: he's ~6", measuring from the anterior to the posterior of the carapace. He was subject to periods of no electricity (thus no basking lamp or filtration, etc) and also to a cold house last winter (barely above 58° most of the time and he had no source of heat other than the lamp). So, I'm surprised he's alive to be honest much less in relatively good health!
He's now been in a 28 gallon tank (I know it should be bigger but it's all I had and don't have the money yet to sink into a 55 gallon tank), which holds about 23–24 gallons currently to allow for the dock so he can't escape (plan to get him an over the tank basking area very soon so the tank itself can hold all 28 gallons and he'll have more water to swim about). But at present he's got a large floating dock (upgraded from a medium dock which was too small), a new over-the-side fish filter rated for 75 gallons, a water heater & thermometer to ensure his water is now warm enough (between 76—78°, yes?), and a heat lamp. Still need to get a light for his UVB needs but I'm quite happy with the improvement in his new setup—the water is crystal clear and warm, he's got a substantially larger dock, so much more room to swim (couldn't even swim before because the water was super shallow), etc.
I suppose my point is just to detail his past vs current environment and that he's a real tough mofo to have lived through all of that... he's been in the new tank now for a couple of months and he has developed a discoloration on a marginal scute of his carapace by his head. I want to know if this is an early stage of shell rot. It was not there when he was in his old tank and before he moved into his new home; I even brushed off the algae on his shell and gave him an inspection to note his condition at the time I moved him to monitor any changes... so I know this occurred recently, and I also believe he's not been able to bask in the new setup. Last night I made some changes to the dock and for the first time in 2 months, to my delight, I caught him basking! So, turtle aficionados of the internet, I'd greatly appreciate your opinions on whether or not this is shell rot or whatever it may be or not be!
I hope I covered most of the bases here in going over the turtle's current and old habitat, but if I missed something let me know... like I said, it's not my turtle and I knew nothing about turtles when I decided I needed to save this little guy, but I'm trying my best now to learn all I can and ensure that I'm taking proper care of him. He may not be my turtle and to be frank
I don't think I'd ever own one myself, but all animals deserve adequate care and attention and that's why I'm seeking opinions before I take him to a vet.
Thank you guys for your advice/opinions/thoughts!
https://imgur.com/gallery/rXAW9