It sound like fungus possibly but difficult to be 100% certain. This us a secondary infection, possibly resulting from a wound caused by parrasites. Certainly the fish are showing classic signs of stress and the lethargy is always bad news.
As Adam has suggested, you need to find out what you are dealing with first. I assume you are 100% certain the water is good, i.e no ammonia or nitrite, no copper, PH 7-9, stable temperatures between night and day, no chlorine or other toxins, runoff for example from nearby plants, decking, wooden or metal structures? Is there enough O2 in the water column, baring in mind the recent high ambients?
If you cannot perform a skin scrape can you get somebody local to do one for you? If not, and having elliminated all of the above as a potential cause, you must consider a shotgun approach. Try M&F first and see if that helps. One of the advantages of this particular medication is that it often gets them moving again which is half the battle. A fish that moves grooms and that helps keep parrasite numbers down.
Will medication harm the filters? Yep for sure. How much depends upon how well established they are and upon water quality. Bacteri favour Lower PH around 7-7.5 and will be much hardier than bacteria living in hard water with a PH around 9.
Best of Luck.
Alan F
Posted on Jun 8, 2006, 1:50 AM from IP address 213.120.101.21