What is that worm?

[Wy Renegade]

I think they have been identified as Detritus Worms. However; 2 months ago I had an African Butterfly and I fed him live blood worms... Is it possible they fell to the gravel and been living there unnoticed ???????

based on the picture, I would say that you have detritus worms, which are a variety of segmented worms, unlike the blood worm which is actually the larval form of a midge (or type of fly). Most likely they came into the aquarium on your plants and then settled into the substrate where they help with the breakdown of organic wastes. I wouldn't worry about them, as was pointed out they are actually beneficial plus they provide your fish with a snack.

 

[bluebirdnanny]

That is better pictures of the worm I was describing to everyone in chat.