The simptons you are seeing can be plenty of things. First of Res in my experience, specialy outdoor raised ones tend to behave in a very freakish way when brough inside for the winter, I frankly beleave that in thouse cases that is depression related thing and one of the things they do is quit eathing and basking to much without being hill. However they do not present thouse simptons you speak about the eyes wich indicates something is furter wrong. I remenbar that in a thread a wille back you said both your res were males, well the biggest one could very well be bullying the smallest one, wich results in stress that could make a imunodepression that results in the simptons your seeing. It is very rare outside something the size of a pond to be able to keep 2 male res together without some kind of conflict going on. At the very least put the sick one on a diferent habitat untill he is felling better (witch is on your top list of prioritys, put the sick animal apart!)
Also the diet your ofering to your clan of turtles is lacking MANY nutrients. First of feeding feeders is a bad idea and they could be the source of the patogens that are afecting your turtle, DONT use them, use fish market brough fish fillets and small rainbow trout. Second reptomin is a very good food but ONLY for SMALL turtles because its like a nutrient injection, full of calcium and protein and its great to make their shells hard and bulk them, however it doesant suits grown turtles such as yours, it has too much vitamins wich arent needed and specialy too much protein, when fed regulary to adult and even juvenile turtles, most either endup piramidized and or with excess proteins in their blood wich ultimatly leads to kidney damage wich is a very serious hillness in turtles. Reserch papers shows it: adult sliders are mainly HERBIVORS. THEY NEED FIBER! I only feed my adults animal protein once a week at the most, reptomin can be used but its a very small part of the diet. Koi pellets, tortoise pelets, chiclid pellets, various insects and inverts (roaches, mealworms, crickets,earthworms, land snails,etc), leafy greens (mustard greens, turnip greens, water cress, hibiscus,etc) with the ocasional human consupsion fish or lean meat is what´s needed to feed your turtles. Mine are more healty then ever and lay eggs every single year!
Back to your sick turtle, the fact that he doesant have any mucus going true is mouth or nouse doesant prove anything, in my experience, in the case of respiratory infections thouse simptons only apear on the late stages of the problem, your turt could very well have a respiratory infection. And that would also esplain the eye prob (mouth, nouse, eyes and lungs are cosely conected). The bad news here is just that there is no way you can treat that at home. Bring your turt to a exotic vet, it wont be waisted money, if its a respiratory infection with the CORRECT antibiotics most turtles make a very rapid recovery and sliders are tough. And now whit the new tips Ive gave you it wont happen again so soon
