It isn't likely that either planaria or nematodes would be responsible for a slime build-up in your filter. That is more likely a build-up of organic compounds that are the by-products of the breakdown of fish waste and uneaten fish food. This symptom, along with the appearance of nematodes, all point to either not enough water changes or a real build-up of 'crud' in your substrate.
-Joe
-Joe
