try it, I don't think it will hurt anything. As long as you don't leave it to rot in the tank, it should be fine. You might want to try cutting them into small, bite-size pieces for smaller fish like the tetras. I just can't see neons taking bites out of whole fruit
Goldfish will, but they're piggies.
Some veggie material is good to clean out their digestive systems. Especially for omnivorous fish like tetras, they should really enjoy the change! Let us know how it goes
just don't overdo the veg's for the betta. They are true insectivores (a type of meat-eater), so too many fibrous foods can lead to bloating. So for my bettas, I just give cut up peas like once every other week just to keep the ol' pipes clear