White threadlike worms found in tank

Question
Hi, recently I have noticed white threadlike worms crawling on the side of my tank or floating/swimming around in the tank and I am afraid they will harm my two green spotted pufferfish. I clean my tank quite regularly and im unsure what is the cause for these "worms". I am highly concerned as I fear that they will make my fish sick and eventually harm their lives. please help!

Answer
Hi Sheena
Thin whitish colored worms, that look like an S when they swim, or look like pieces of hair floating in the water-if that sounds like them, then that's just nematodes.  Harmless to fish and humans so don't worry. In fact, most fish will eat them, and I'm sure your puffers are as well.  I have them in my tanks, and even transferred (not on purpose) some into my tanks with no plants from nets and filter media and such.  I've seen my zebra danios and dwarf platy fry chase after them and eat them.  

Usually they'll hang out in the gravel/substrate, and most people notice them after doing a gravel vacuum and disturb the gravel.  They'll usually hitchhike in on live plants or snails, or even with the pet store water.  They'll thrive in tanks that the fish are being overfed/lots of food left over in the bottom, and/or in tanks where there's not enough water changes/gravel vacuums being done.  To help get rid of them, cut back on feedings, only feed once a day or less, don't leave uneaten food in the tank-remove it after a few minutes, and weekly water changes/gravel vacuums of at least 25%(depending on tank size and stocking level).  Like I said, they're not bad, just want to keep them in check so you don't get a gazillion of them, because then you could be facing some water quality issues.  Usually when you see worms or other critters floating around in the tank, they're harmless.  If they were "bad" parasites, they'd be on or in the fish already.  

Hope that helps!

Christy