Depends what fish you keep,my fish will eat just about anything.
In the summer i use mosquito larvae which i collect from little pots i leave out and i use earthworms that i collect off the lawn early in the morning all year round.
In the wild fish eat a lot of insects that either live in the water or fall in off of bankside vedgetation so just choose insects that your fish can manage and try them.
I put one in and my black tetra ate it. I just didn't want to put anymore in incase it is abd for them. I asked around a couple forums I'll see what responses I get
I tried putting some ants in but my fish (tetras, platys and gouramis/ didn't really like them. Of the three I put in, two drowned and one got pecked to death. Took him while to die - it wasn't nice.
Elisabeth i dont "mash" the worms up but i do cut them into inch long sections with sissors so that they are easier for the fish to manage and they cant bury themselves into the substrate.