Good advice above. There can be different causes, including a sort of fish tuberculosis. When I've had fish with that symptom it was normally confined to one fish, but it sounds like it's more than that in your tank. Prazi Pro (or other praziquantel med) is a dewormer and adding Metro works with other pathogens like flagellates and some type so bacteria. Neither one should bother the filters.
If you read down a ways on
this page, this discus breeder has a whole little system for internal problems. Some of the info might be helpful, in terms of his approach.
...Come to think of it, Larry has posted some good information in a similar vein.
Treating wild fish
This post has been edited by neutrinoman: Mar 9 2010, 01:22 PM