The diet doesn't sound bad, you could try adding some other brands of food to the diet. I don't have a DBT but I do have aquatic turtles, I vary the diet as much as I can with fresh foods & commercial foods both because IMO variety is good for them.
Mazuri makes a good aquatic turtle pellet, I also use frozen fish foods for mine such as blood worms, brine shrimp, one called emerald delight (but I thaw them out first.) I use a variety of live fish & insects as well.
This new Ectotherm brand of Turtle Yummies seem to be a good treat. I found that the other day & tried it, they seem to like it.

It could be that the one eye is injured, is there a basking area he could've bumped into? Water changing shouldn't cause an eye problem but it is possible that he has some sort of eye infection.
You've heard of "pink eye" in humans & other mammals I'm sure, well that sort of thing can happen to reptiles too. Bacterial infections can just happen because bacteria is all around us.
If it's an injury that doesn't get infected it should clear up pretty quickly but if it doesn't clear up soon I'd still see that vet.
Good luck with this, let us know what happens!
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PHRatz

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