yep deinitely planaria. I had this (and still do a little) in my
10gallon tank. Toni's advice is good. If you have any plants in the
tank, remove them and give them a damn good wash. Also wipe the insides
of the tank and then do a huge water removal.

If you can move all your fish out of the tank into another, you
could always try raising the temperature to about 105degrees F for
a day or so as this will kill the little gits too. Unfortunately for
me, this wasn't an option, and I still see the odd wriggly planaria worm
in the tank. They are harmless but if you have LOADS they can get into
the fish's gills and irritate them.