My fish tank

Question
Hi,
  I was wondering why when i went to change the water in the tank i found a worm wriggling, it was brownish in colour and about an inch long? Is it harmful to my fish?
And why is my internal filter after i washed it, the sponge turned a pink colour, is it the food i am feeding the fish which makes it pink? How can i get rid of the colour?

Answer
Hi Brendon,
95% of the time, any worms that are found outside of the fish swimming in the water or living in the gravel are non-parasitic and harmless.  That brown worm is most likely harmless.  Is this the only worm you have found?  Maybe those white "strings" you found before are smaller worms and you are mistakening them for non-living objects.  Try to find the source of the worm and eradicate it.  

For the sponge turning pink, it could be something the filter is filtering out of the water such as fish food or decoration paint.  Or it could just be the beneficial bacteria and diatoms.  If your fish is fine, chances are its nothing harmful.  
-Matt-