I'd be surprised if one guppy caused such a change in his eating behavior. A bit of sardine now and then as part of the diet is OK. Rinse it off if there's oil on it (sardines packed in water are preferable). A diet of mostly Can 'o Shrimp and krill is really high protein and not that balanced.
Hatchlings/younger turtles do need a bit more protein for growth, but still need the nutrients that other foods provide as well (including plant matter). Have you tried leaving a strand or two of anachris (an aquatic plant you should be able to buy in an aquarium shop or petstore that sells fish) in the tank for him to nibble on? Try some of the live foods, particularly the earthworm (cut it into pieces if it's large).
