worms in fire bellied toad water

Question
Hi, I have had two fire bellied toads for about two years.  Today as I was
changing their water, I noticed skinny little black worms swimming around in
their water.  I've been researching and researching what they could be and
can't find anything.  The toads eat crickets and have never eaten any kind of
worm so it cant be larvae from left over food.  I was looking to see if it's some
kind of parasite.  I'm not sure what else it could be.  This is the first time I've
seen this!

Answer
Huh,  that's got me stumped.  What type of enclosure decor are you using?  Is there anything that is a natural decor and not man made?  It is possible that it came from something collected form outside that was added to the enclosure before being treated.  What I would do is break down the whole enclosure and clean it then put it back together with some of the water that has been filtered so there are no worms and wait and see what happens.  I don't know of any black worms that are a parasite that would just show up like what you describe.  If you can photograph them I may be able to contact some others in the industry that I know and see what we can do to identify them.