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My turt appears to be sick so I figured vitamin C might help his immune system. What turtle-friendly foods have good amounts of vitamin C in them?

Turtles make their own Vit C, so I don't think you need to worry about supplementing if he's got a good diet. If he's sick, boosting his immune system by increasing temps and getting some Vit A into him (can be done safely with foods high in beta carotene like carrots, dandelions, squash, red peppers etc.) are especially helpful.

What kind of symptoms does he have?

squirt is making the faint squealing sound (i think its sneezing?), he is listing to his right side, and he has a small dark circle around his nostrils (is it possible to have a nose abcess?)

...i am aware the above symptoms are characteristic of an RI. squirt actually just finished a round of antibiotic treatment for this respiratory infection so im debating on whether i need to go back to the vet and get more... i'm only 14 so i need to 'conserve' the number of times i go to the vet... but i think it would be safe to do so just in case.

When he finished his round of antibiotics, did he appear fully recovered and now has begun showing symptoms again (from what you said it does sound like an RI)? Perhaps the infection wasn't fully knocked out of his system. A black ring around the nostrils doesn't sound like an abcess (no swelling or lumps anywhere?). I understand your situation about "conserving" the number of times you take him to the vet, but it really is better to be safe than sorry (I know you already know this by what you wrote).

Is he eating? What is his water temp and basking area temp? Is he in a room with AC?

He is not in a room with AC but he is by the front door, although a wall does separate the two. Water temp is now 82, basking temps are probably around 90-95. After antibiotics he did appear fully recovered for a few days, then showed the slightly lopsided swimming, bubbles/open mouth breathing underwater, sneezing, etc. So i dont think the infection fully went away. I'll take a picture of the nose circle thing later on tonight; I believe his nose maybe has mucus or something that forces him to breathe mostly from his mouth. Perhaps it is normal after treatment (like how humans have runny noses after colds sometimes), but I will talk to a vet to confirm.

Thank you again for all your help!