Planaria aren't harmful, really. They're pretty good foods for fry, but that's because they're moving and squirming, and it gets their attention.
If you'd rather not have the intruders there...
After maybe two or three weeks of water changes, I got rid of my infestation. It was pretty bad, too, since the hairgrass and sand hid them all pretty well. You could completely empty the tank and wash it out with hot water, then refill it and etc., but seeing as they're not harmful and you've already got everything pretty set up, you might not mind just doing the regular water changes.
Edit: I also do 50-75% water changes for my 5g. (I don't mean to get that much water out, but the syphon sucks. lol)