So, I scrubbed him with mild detergent and a toothbrush, which loosened lots of scutes. As they came off, lots of algae and crud came off too. It peeled and peeled and peeled.
After I peeled a lot off (I didn't peel anything that did not come easily, I didn't want to go overboard), I dried him thoroughly with a towel, rubbed Tea Tree oil all over his carapace and plastron and put him in dry dock for the night. The next day I could see that more lifting was happening, but he looked much better, so I put him back out in his tub that morning. Thay night I let him stay out, but repeated the process the third night.
His shell is looking a lot better and so for now I am going to leave it alone and keep an eye on it. But I was amazed by how much keratin has built up. I am not sure what is causing the buildup. His nutrition is good, he basks regularly (we had 90' temps for the first couple of months he was outside, then temps in 80s, and now temps in 70s), etc.
Is it possible that moving outside and getting increased UV could be accelerating his shell growth? I am grasping for reasons that he would be retaining like this. It was so severe that his colors were kind of distorted. With a lot of the buildup removed, he looks a lot more like he should now. Because it happened to gradually and he is outdoors now, I did not notice it until it became really severe.
I am going to keep an eye on it, but if it doesn't start shedding soon I will dry dock him again to jump-start it. The tea tree oil seemed to help.
Here is what his shell looked like before treatment (icky buildup!):
And here is what it looked like after treatment (big improvement, but still a ways to go):
(The whitish-looking spots are not shell rot, they are places where the scutes are lifting and air is beneath them.)
Anyway, I thought this would be informative or at least interesting. And if you have any thoughts as to what may have caused the retention, please share!
