fresh water aquarium fish

Question
When I move around gavel at bottom of tank debris fills tank. I see very small hair-like worms or parasites that wiggle around. My guppy eats them. This occurs in my 5 gal isolation/fry tank. What are they and are they harmful to my other fish? They live under the gravel.

Answer
Hi Paul
Those sound like nematodes.  They're harmless to the fish and humans.  They usually show up in tanks where there's lots of uneaten food in the gravel.  Do more frequent water changes being sure to get down good in the gravel, and usually it's recommended to cut back on feedings, but with fry I wouldn't do that.  
Plus, like you said, some fish will eat them.  I have it in  one of my tanks, and transferred it to my 10 gallon fry tank when I moved the filter media to that tank.  I laughed when I saw this itty bitty dwarf platy fry with this worm bigger then him sticking out of his mouth lol.  

You could also remove some of the gravel, most places say you have to have an inch of gravel in the tank.  But really, if there's no plants in the tank, you only need to have about 1/4-1/2 an inch.  Actually, you don't need to have any gravel/substrate if you don't want it, mostly just for decoration purposes.

Hope that help, and let me know if you have more questions.

Christy