Hi there!
My husband is a teacher and one of his coworkers is retiring to another state. He offered us his classroom turtle (plus the whole setup) so we said yes.
I know it is a red-eared slider, and that the teacher was trying to give it a "pond-like" environment. I know there were two in the tank, but one died earlier this year. I have no idea their age or where they came from.
Based on what I can find from google, after we got him (I'm guessing him. We named him Turkleton, since he had no name before) I cleaned a bunch of poop (like, years possibly of buildup) out of the gravel (I've seen that gravel is bad, we will work on that later), changed out most of his water now (the advice we got was only change a little at a time, I've been feeling like I'm rushing it but he was swimming around in green water.), almost doubled how deep the water is in the tank, got him a few more basking rocks and I've been working on starting to vary his diet - right now he is on pellets and dried red shrimp, around 10 pieces every other day.
I checked out the RES at the pet store and now I'm concerned about his health, I'm hoping more experienced eyes can help me decide the next steps with him.
Here he is
Here is the tank (based on measurements, around 32ish gallons)
(I know a window isn't ideal but this is the only spot in the house we have for him)
We have a good filter, a water heater (we turned it off because he got really lethargic and stopped going in the water, I'm working on getting a thermometer), and a light (no idea what type or anything about it) that we haven't tried to use yet since he is by the window. He doesn't get direct sun since it's summer, but he does get lots of light. In winter he would have direct sunlight.
My concern is his shell, and that he seems to be shedding skin. You can see the skin in the picture of him (shedding started at the front legs and neck, now his back legs have the flakes), but I'm less concerned about that then his shell.
His shell doesn't feel soft, I'm not sure why it's all black like that. My first impulse is to try and wash him, since it seems to be some kind of build up, but I'm not sure how or with what.
I've looked up a little on youtube and tonight I might just try some gentle scrubbing with a toothbrush and water to see if it comes off, but the guy at the pet store said it might be shell rot so I'm concerned about that.
I'd also just love any general advice - I've never owned a turtle or reptile. My current plan is to get variety in his diet (romane lettuce, carrots and turnips probably), get a thermometer, do a bit of a shell scrub with water until I get better advice or have more time to research, and keep working on cleaning old poop out of the gravel. Thank you so much in advance, I know this is a lot to ask!