Brian,

I've been hit with Planaria in my tank recently. They can be as
little as a couple of millimeters and as long as a centimeter
or more. They typically look like white hair floating in the tank
but when they swim, the wiggle.

They are caused by overfeeding fish, high ammonia, or brought in
from plants.

A couple of ways to "attempt to kill" them (found this on another news
group)

1) Stop feeding the fish for a few days
2) Remove all fish and raise temperature to 110 degress farenheit
3) Add Apple Snails (or similar)

So far I've tackled my problem by "fasting" my fish for 4 days
adding some salt (1 teaspoon per gallon) and doing a 50% water change
and doing a very thorough gravel vac (and stirring the tank and
vacumming any debris from the water)

I still have a few, but nowhere near as many as I had before.
They tend to hide quite well. I initially thought I only had a few
but when stirring the water I found I have quite a few indeed.