I'm no expert but they just like garden worms to me, shouldn't be harmful but might rot and cause an ammonia spike/higher nitrates.
Like I said, not an expert but thats what they look like to me, I'm guessing you fed them to your fish, but they ignored them and they died in the tank.
Where are your blackworms originating from? I'm just about to give mine the nightly rinse & feed routine, I've got 10 more pounds coming in Thursday from California Blackworm. I'll take a closer look, but I haven't seen anything like that in them yet.
they are prob about 3-4cm and I havent fead them live food for at least 12months, could they have come off plants?
I did put some in a few months ago and I'm talking upto 6 months.
If you have fed any live blackworms, they could be some that escaped into the substrate. They will often live for some time there. The size is about right for them.
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Hi Tolak
My corys are small, and while they eat a lot of worms for their size, it doesn't pay for me to order them online and risk shipping problems. I pick them up at the lfs, so I don't know their origin.
My blackworm supplier finally got back to me on this;
Hi John
Sorry for the delay response, I was out of Calif.
Those larger worms are in fact a type of earth worm.
Most larger fish will eat them along with
the blackworms. They are around but not in large
numbers just a few here and they, Their really
nothing to worry about.