A lot of fish specie love to eat worms, and it is also a natural and protein rich source of nutrients known to make them thrive. This doesn’t mean that you can let your fish have free access to worms all day long; worms tend to be quite fatty and eating to many worms can lead to obesity. The ideal worm type depends on the size of your fish (unless you want to cut the worms into smaller pieces). Fairly big fishes are for instance known to love black worms and red worms, while small species and fish fry appreciates the small grindal worms.
If you have both small and big fishes, you can set up a combination worm culture. A combination worm culture is also a great idea when your big adult fish produces small fry. One example of a combination culture known to work is the Red worm + Grindal worm combination. Some aquarists even report having greater success with Red worm + Grindal worm cultures than with Grindal worm cultures alone.
If you want to set up a Red worm + Grindal worm combination culture, you will need a plastic shoebox with a tight fitting lid or a similar type of container. Fill the container with 50% fine particle top soil and 50% ground cellulose worm bedding. (Worm bedding can be purchased at well-stocked pet shops and aquarium stores.) Ideally place the container in a damp spot, e.g. on the basement floor close to an outdoor wall, since the worms want the soil to be kept really moist.
Both Red worms and Grindal worms like to eat one-minute oatmeal and you should sprinkle it over the surface every 2-3 days. Adjust the amount of food to the size of your worm colony to avoid left-over food that can produce a foul smell. If the oatmeal has not been devoured within 36 hours, you are giving your worms to much to eat. As a general rule of thumb, a thriving culture will eat one handful of oatmeal per 2-3 square foot surface area within 24 hours.
Grindal worm harvesting
When it is time to harvest the Grindal worms, sprinkle oatmeal over the soil surface and wait 12-24 hours. The Grindal worm will migrate up to the surface to eat and you can use a spoon to scrape them off. Separate them from soil and food by rinsing in cold water 2-3 times, letting them settle and removing soil and food between each rinse. Use an eyedropper to serve the harvested Grindal worms to your fish.
Red worm harvesting
Red worms are much bigger than Grindal worms and you can simply use your hand to pick them from the surface. If you dislike touching worms, use tweezers.