Please bear with me through this paragraph, but I'm really at the end of my rope with this. When I came home from Thanksgiving dinner, I noticed all of my goldfish just sitting on the bottom. As soon as they saw me, they immediately came up for food, but soon settled back into inactivity. Yesterday, I tested all the params in triplicate, and came up with nitrate between 0 and 5, nitrite and ammo at 0. Performed a 50% water change anyway, and went to bed. Today, they are exhibiting the same lazy behavior. They are still feeding greedily, but seem to bottom sit most of the rest of the time. Weirdest thing is, over the past year or so, this is now the third time that my goldfish have shown a sudden unexplained decrease in activity. All three times, there has been no loss of appetite, which first, led me to belive it was flukes. I treated with praziquantel, and the fish got better. A few months later, it happened again, and again I treated with prazi. This time however, it didn't seem to have any effect, and after one of the fish dropsied up (immediately quarantined where it eventually died) I decided to treat the whole tank with kanamycin and metronidazole treated food. Again, the fish rebounded, and it's been about 4 months of good health and growth since then. The tank in question is 75 gallons, and holds 3 (used to be 4 till an unfortunate siphon accident last week) goldfish ranging from 4 to almost 7 inches. Filtration is a rena XP3, XP2, and a fluval 4 internal filter. No new fish, plants, snails or anything have been added for months. Again, sorry for the essay, but I really have no idea what I should do at this point. Any input would be greatly appreciated.