I found a worm in my fish tank today! and its alive!?

THREE MONTHS AGO i put a worm that i found outside in my ten gallon tank just to see if my fish would eat it. They nipped at it on the way down, then ignored it once it started burrowing. This morning it was on top of the rocks, burrowing back into the gravel!! He is still alive, and he's gotten bigger! Can anyone tell me how this is possible? Ive heard they only live for 11 days submerged. Its a freshwater ten gallon tank. 76 degrees F. Maybe 10 fish. Tetras, mini cat fish, one large sucker fish. I have a bubbler, so thats adding extra oxygen..i guess. And a filter. Of course. I feed tetra min fish flakes, and the ocasional spoonful of blood worms. And alage wafers for the big sucker fish. Will the worm hurt any ballance? Do you think he'll eat fish poop? I like having him in there, its neat to me. How do i keep him alive?