I figured I'd start a new thread on this... Others have suggested I may have internal parasites in my tank. This may explain my sudden losses of fish that exhibit no particular 'symptoms'. (I had some previously poop white) many seem completely fine one night and then are dead the next morning. Here's the thing... I just recently restocked my tank since so many had died. I only have about 3-4 fish left from my previous stocking left. (I have a 55 gal, 2 guppies, 4 platties, 6 tetras, 1 molly, 1 pleco) (only the 1 guppie and 2 platties are original). 1) How contagious is internal parasites? Is it passed from fish to fish or is it present throughout the tank and any fish could get it? 2) Others have recommended medicated food. Is it worth treating my whole stock at this point with medicated food? As I doubt the new stock are diseased... unless it's terribly contagious... Any thoughts would be helpful! Thanks! *As a side note / question... My tap water is not usable for water changes (high sulfur content toxic to fish, slight ammonia reading in it). I also can't go buy 55 gal of freshwater each time. So I go to our local spring and get water from there. Is it possible that the parasites are being picked up from this water source, or are parasites something that kind of grow through the bacteria in the tank? Is there a way I could test my water source for anything but the standard ammonia, nitrites, etc etc etc?