Hi Chris,
I haven't heard of people successfully keeping Chinese firebellies (
Cynops orientalis) on a feeder fish diet (though I have no evidence that to suggest it's unwise).
Be aware that the presence of fish in a newt tank, depending on the size of the tank and the number and kind of fish, could be a stress factor, especially if you notice your newts seem reluctant to enter the water. But you did say they're in the water most of the time. Also, if it's not a well-cycled tank with adequate filtration, the fish excrement and uneaten fish food could also cause water quality to wane faster than if there were just newts in there
And feeder fish are bunched together for long periods of time before you actually buy them, so they could carry diseases and parasites that are easily passed from one fish to another in those unnatural conditions.
As Morg said, firebelly newts will generally do fine on live or frozen fortified bloodworm, and chopped earthworm (for those people who are into chopping up live worms
). If your newts turn their noses at the cheapest frozen bloodworm, which is generally low quality, try the high quality frozen kind. Live bloodworm is great, but there again you run a risk of introducing something...
My Chinese firebellies, when they're on land, will sometimes turn down larger wax worms, giving the impression they're uninterested, but will then sometimes eat smaller ones placed in front of them. It sometimes takes patience and persistence (and you shouldn't distract a skittish newt by hovering over it all the time). If they don't eat one day, try again the next and you might get different results!
If your mostly aquatic animals are not eating, you could set up a small container just for feedings. Add one newt and a few bite-size crickets in there, and wait a few hours. Then count the number of crickets left to see if this method is effective. Don't add so many crickets that your animal is overwhelmed, and put some moist moss in there and maybe a tiny shelter too to relieve stress. In an open space with oversized crickets wandering all over the place, the newt will think only about escaping, not eating. Many newts need to be relaxed to eat.
Are you keeping the toad separately?