One of the best planaria eaters is the paradise fish.
You can also trap the planaria. Take a firm, trimmed piece of liver, wrap it in a piece of cheesecloth, and put it in a wide-mouth bottle. Drop the trap in the tank as you turn the lights out. Leave it in the tank for a couple of hours. Later, cover the opening of the bottle and remove it from the tank. The cheesecloth should be covered in planaria. Do this several times with a fresh bait each time, and you should be rid of the worms in no time. Planaria reproduce both sexually and asexually. They can reproduce by dropping their tails and the head will grow a new tail while the tail grows a new head. These hermaphrodites can also lay summer and winter eggs. So, you may need to use the trap again well after you think the planaria are gone.