Hey got a softshell huh?
If you can get us some pictures of it , we can help you identify it so you will know exactly what species it is. Perhaps you can browse the galleries of softshell photois here and try to figure what kind it is for yourself.
If it is an american softshell, crayfish are a preferred food item. Some people feed only cooked or frozen (thawed) crayfish to teir softies to help avoid parasites.
As far as goldfish go, they are really a poor choice. although they are readily available they are among the most mistreated animals in the pet trade. They are often heavily parasitized (which can mean parasites for any turtle which eats them) and they are usually NOT even fed at pet stores or wholesalers and they are often raised in water which has been trated with large amounts of copper (to kill algae) which builds up in their bodies and will become very toxic to animals which eat them.
You can raise your own goldfish if you like or try to get bait store minnows or mollies from a pet store.
You might also try superworms (assuming the turtle is at least 5 inches long) wax worms and crickets as insects make up a large part of many softshell diets. earthworms as suggeste d above are a good choice as well.
Good job finding a new home for the red eared sliders, other turtles kept with softies tend to scratch their sensitive shells, so it is best to keep them alone.
As for the depth of the water, i would suggest trying to make it shallow enough that the turtle can easily get to the surface without swimming. Softies like to rest on the bottom or in the substrate and come up for air by reaching their necks or doing only a little swimming. They may become stressed in deep water (though they ARE excellent swimmers and some keepers do house them in deep water).
Try to use either very fine sand or very large smooth gravel as softies tend to get scrathed by sharp rocks and may eat small ones by accident when they are eating their food. So get rocks that are smooth and bigger than their head or sand which is very small.