Hello!
I’m new to this forum & looking for guidance with my RES. He’s an adult, I’ve had him ten years from the size of a coin to what he is now. He’s always been... green.. for the most part.
Over the past couple of months, notably since around the time I moved, I’ve seen his top shell transition from it’s usual green to a more albino, yellow hue... more predominantly near his head.
When I first noticed this I panicked and assumed shell rot, but upon inspection I couldn’t find any abnormalities in his shell. I cleaned and quarantined him until I could take him to a local (supposed) reptile expert who, to my surprise, told me it was the healthiest RES he’d ever seen.
I took him home and a couple months have since passed, but the colour transition hasn’t slowed or stopped and I’m again concerned I might be overlooking something to with his health or habitat that’s causing this? I have him in a 18x48x24 tank with a heater and two lights, uva and uvb, with a fluval 406 canister filter, and an ample basking area that’s usually in the 84-86 degree range. I do use water conditioner and tested my water’s ammonia, nitrate, chlorine, ph, nitrite, alkalinity and hardness levels with store bought strips.. everything appeared normal apart from a spike in the alkalinity.
It’s tough to find advice re: turtles and I’d greatly appreciate any advice or pointers fellow turt lovers have to give.
Thanks!

