Just thinking about it gives me goosebumps. It looked like an earthworm, but smaller. Maybe about an inch long, and very visible! Brown and slimy. QUOTE]
Its an aquatic worm. Totally harmless and wonderful fishfood. I once paid $35 for a shipment of "California Blackworms", which are probably "kissing cousins" of what you found.
I would consider aquatic worms, which aerate the substrate, a big plus. The only problem they might cause is a little turbidity as they stir up the soil. But generally, they don't last long in tanks as the fish go crazy for them.
Creature in photo is a flatworm, maybe some kind of planaria and totally harmless. [Calling a biologist for identification...]
Please folks, an NPT represents a natural ecosystem. The tanks are naturally going to contain all kinds of harmless little critters, including aquatic worms, planaria, hydra, etc.
The only disease that I know fish hobbyists can get from their tanks is "Fish Tank Syndrome". This disease is due to mycobacteria, the bacteria that causes "Fish TB". I know a few, very experienced hobbyists that have gotten the disease, even though their tanks were not having any disease problems. One hobbyist got the disease from a small splinter on her finger; she was going to have surgery to remove the painful, large sore from her finger until another hobbyist suggested the sore could be due to a mycobacterial infection, and was therefore, treatable. Thereupon, her doctor consulted a fish veterinarian, canceled the surgergy, and treated her (successfully) with the appropriate antibiotics.
Recommended precautions against "Fish Tank Syndrome": You should not clean tanks if you have an open wound. Always wash your hands and arms after cleaning tanks. If you mouth-siphon (not recommended), rinse your mouth afterwards with water. If you are immuno-compromised, I would double-up on the precautions.