I don't think I would feel comfortable feeding worms as a staple diet. As a supplemental item I could see it being a great addition to my feeding schedule though.
For most fish I would not recommend a complete diet of worms either but copperbands can get most of their food from worms as that is what they are supposed to eat. They need a little of other things to get calcium, iodine etc. but a copperband should have worms.
Crickets, beetles, mealworms etc are not a good diet for salt water fish. They do not have the proper oils and their shells may not be dijestable as they are made of chiten.
(When I started keeping fish in the 50s and 60s fish food was dried ants in a can.)
I do not buy earthworms but when I used to feed earthworms I would just collect them. They could also be frozen. If you live by the sea you can buy (or collect) saltwater sand worms or blood worms. These worms are perfect food but they are large and need to be cut up. They also freeze well. They are about fifty cents each and grow about 9" long.
For me, live blackworms are perfect but I realize some people can't get them.
It is not the fishes fault if you buy it and do not have the proper food for it. It was happily eating something for a few years in the sea and does not recognize the foods we are trying to feed it in a tank.
I have a tank full of butterflies that I collected. They only eat worms. I wish I could wean them to something else but they are being very stubborn, in the meantime, I have to give them worms. If they don't convert to something else, I may return them to the sea next summer.
Most of us feed our fish commercially prepared foods because it is convenient and many of us have nothing else but sometimes we can get creative.
Commercial foods are sold to make money. They are pckaged for the convenience of the consumer and the producer but it may not necesarily be the best thing for the fish.
Have a great day
Paul
PS do not gut load worms