Most fish don't eat planaria very readily, or unless desperate for food. Putting more fish in will necessitate more feeding, which will likely increase your planaria problem rather than combat it. They don't seem to harm fish but they will do severe damage to fish eggs.
Somebody (not sure, but I think it might have been
Trouser Cough
) recently mentioned a product that apparently works well to get rid of these critters. I can't remember the name of the stuff, something goofy like Planaria-Be-Gone or an equally unimaginative moniker.