California Blackworms (Lumbriculus Variegatus), most commonly referred to as just blackworms, are a popular tropical fish food. In addition to feeding them to my betta fish, I have used them as the primary food for my Dwarf Puffers and Gouramis. These worms are fed live and while they can be impossible to find at large chain pet stores like PetCo and Petsmart, they are often sold at smaller private fish stores or purchased online.

Blackworm Benefits

  • Many carnivorous tropical fish, including bettas, eat worms in nature. When they see the wiggling worm in their aquarium they attack it with fervor, hunting it down and sucking it up like spaghetti. Blackworms offer both nutrition and enrichment. Even finiky fish find themselves too tempted to turn up fresh squiggly blackworms. Dwarf Puffers, which are supposed to be round and fat, rely on the worms to keep them healthy or risk falling underweight. Betta splendens, often picky eaters who spit out flake or pellet food, will gobble them up and beg for more.
  • Blackworms are inexpensive. The store I bought them for a couple of (U.S.) dollars per scoop (a scoop was like a large tablespoon). One scoop could feed 3 or 4 small fish for a week or so (maybe more).
  • Blackworms are easy to care for. I’ll get into the specifics for caring for them in a moment, but generally speaking, they need little more than a cheap Tupperware container, clean water and a refrigerator.
  • Blackworms are sized right. At about 1” – 3” (2.5cm – 5cm), blackworms are already portioned out perfectly for small tropical fish. I allow my Dwarf Puffers to gorge themselves, since keeping their weight up is a challenge. Those little guys can eat several in a sitting. I just pluck one out with tweezers and drop it in the tank. The worm is as good as eaten in moments. I’m more judicious with my betta fish, but even with them I throw the rule of thumb on portion size out the window. (The rule of thumb being that you should feed your betta fish the amount equivalent to one of his eyeballs, twice per day.) Since I’ve never had blackworms cause constipation or bloating, I usually feed them two or three good sized worms twice per day. I don’t know why bettas don’t have the same digestive issues with blackworms that they do with manufactured food. Perhaps it is because blackworms are mostly water and are easily digested.
  • Lost blackworms won’t foul your tank. Betta fish, in particular, will often let food float past them to land, and ultimately rot, on the aquarium floor. Blackworms will occasionally get past the slow swimming betta and will burrow into the sand below. They can live a long, healthy life there, aerating the soil and providing a healthy substrate for your aquarium plants. Often they will stick their bodies half way out to feed and along a fish will come and gobble them up. It is quite a site to watch a Fighting Fish scour his tank in search for food and attempt to pluck a forgotten blackworm out of the sand. The tug-of-war between the two is impressive and it offers quite a lot of excitement for the fish as they fight to wrangle the worm.


Blackworms, originally uploaded by Nymsley.

Preparing Blackworms

  1. My local fish store sells blackworms in a fish bag with just enough water to cover them. The rest of the bag is filled with air (like when you buy a fish).
  2. When I get them home I immediately move them into the WORMS-ONLY Tupperware. It’s a small round bowl with a watertight lid.
  3. Some people say that you HAVE to wash them with conditioned water or the chlorine will kill them. This has never been the case for me. Because I like to do things the easy way, I simply rinse them in cold tap water. I store them in that water too and have never had a problem with the water killing them.
  4. With the worms in the bowl I fill the bowl with the coldest tap water I can. As the summer progresses, I often find that I can’t get very cold water from the tap, but the difference in temperature doesn’t seem to shock them.
  5. With the bowl full of water, the healthy live worms sink to the bottom. Dead worms and worm parts float. I carefully pour off the excess water and let the dead worms run down the drain.
  6. I repeat buy filling the bowl with more cold water, swishing the worms around to loosen any dead worms from the tangle and again pour off the floating dead worms.
  7. Once I no longer see any dead worms (there are very few when you buy a fresh batch) I fill the bowl with just enough water to cover them. I close the water tight lid and store them in the refrigerator. I don’t have a separate refrigerator for fish food so I just dedicate a drawer to them in the bottom of refrigerator. Even after all these years, the worms still gross me out enough to not want them sharing a shelf with my food. If you’re not squeamish, this worm-quarantine isn’t necessary. I just don’t want to see them when I’m searching for a snack.
  8. Twice per day I repeat the process of washing the worms in clean, cold water. I do this just before I feed them to the fish. When you first remove the blackworms from the refrigerator, you will notice they are twirled together in a tight mass. This is normal. Keeping them refrigerated, I’m told, will prolong their life. I don’t know if this is true, as I have never kept them any other way. I do know, however, that they will survive long periods in the warm aquarium water as well.
  9. If the water is kept clean, the blackworms will easily live at least a week. By the end of the week the water tends to get cloudy faster, more worms turn up dead, and they tend to get a little thinner, since I don’t bother to feed them. Continue to throw away the dead worms so they don’t rot in the water with the live worms. Never feed dead blackworms to your fish, as they may harbor bacteria that could cause illness.

Feeding Blackworms

Feeding blackworms is easy. For small Betta or Dwarf Puffer aquariums where there are usually just one or a few fish at most, worms can simply be dropped in front of the fish with tweezers or a pair of forceps one at a time. For large community tanks, a feeding cone can be filled with a bundle of blackworms and fastened to the glass with a suction cup. They will stick their heads through the slats in the cone and will be plucked out by fish. Bottom dwellers and skittish fish who won’t compete with more aggressive fish for food should be hand fed using the tweezer or forceps.

Blackworms are a wonderful food for your fish and are a great choice for fish keepers who don’t want a lot of hassle.

Post Rating
1 Star2 Stars3 Stars4 Stars5 Stars (1 votes, average: 5.00 out of 5)

Reader Interactions


  1. Laura-Ray says:

    I’ve had great luck with blackworms in my terrarium/aquarium mix tanks. I used to have a fire bellied newt and a betta in a 10 gallon tank- half of it was land, half was water. The two coexisted very happily. Every month or so I put in an ounce of blackworms. They would bury themselves halfway in the gravel, and the newt and the betta hunted them every day. As long as I had a filter, everyone was happy and the blackworms flourished.

Leave a Reply

Your email address will not be published. Required fields are marked *