Turtles love to snack on all the things you've suggested, fish, crayfish, insects, vegetables.
Meat
Fresh water turtles will happily feed on insects, spiders, fish, crayfish, earthworms and meal worms. A regular live fish as a treat is a good idea. Preferably the type of small (or young) fish that the turtle would naturally encounter. In this case freshwater fish.
The Australian Freshwater Turtles gives a comprehensive list (1) of fish suitable to feed fresh water turtles.
Sword Tails (Xiphophorous hellerii)
Mollies (Poecilia sphenops)
Platys (Xiphophorus maculatus)
Common Guppies (Poecilia reticulata)
Neon Tetras (Paracheirodon innesi)
Gold Barb (Puntius sachsii)
Flat-headed gudgeon (Philypnodon grandiceps)
Carp Gudgeons (Hypseleotris spp)
Gambusia (Gambusia affinis)
(this list is compiled from The Australian Freshwater Turtle (1))
Plants
The World Chelonian Trust has some extensive information for nutrition for turtles (and tortoises) (2), I have created a brief summary:
The vegetables and plants that are well recommended for providing good nutrition for your turtle are: chicory, raw-collards I, dandelion, duckweed, endive, grape leaves, kale I, kelp, prickly pear fruit and pads, turnip greens and watercress.
Iceberg lettuce has no nutritional value for turtles.
I High consumption of cabbage, broccoli, kale, collards and the like can lead to thyroid problems (this will not always be a problem for turtles, but can be).
This post How can one care for a small snapping turtle? also gives good feeding tips for turtles, that would be helpful for your yellow-bellied slider.
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