Guppy Problems

Question
I have currently 2 adult (1 male, 1 female) and several baby guppies and a "mystery" snail and many smaller plant snails in my 20 gallon tank. I have a waterfall-type external filter and the temperature is around 76-80 degrees F. I had recently bought another adult pair, but they died recently for no apparant reason whatsoever, just seemed a bit lethargic one day, and in a short time after that, they were dead. And now my fry, which I have free in the tank with the adults, who do not really try to eat them, seem to be falling ill as well. One larger fry died just today, after seeming healthy just this morning. Another has an abnormally swollen abdomen. A couple seem to have a white, cloudy eye, others look a bit too thin, and most appear to have a thin layer of whitish stuff right on top of their heads, but it is only visible from a side view; it is invisible when viewed from above. I have a fish book with an index of common diseases, and just can't seem to figure out what is wrong with my guppies. They seemed fine until I recently bought another pair of guppies for the tank, which, as I mentioned above, died recently. Maybe they introduced this disease? Do you have any idea what it is? The white stuff on the heads of the fish does not rise very far from the surface of their skin; it is hardly noticable, not like a full fledged fungus invasion. I recently treated them for white spot with Nox-Ich, which contains Malachite Green. Could that medication have adversely affected my gups? (The ich is gone now, by the way) If you can identify this disease from what information I have been able to provide, can you advise me of what way I should best treat it?
Thank you,
Megan.
By the way, I don't think it is dirty water, as I changed the water and filter only a couple weeks ago, and I did use a gravel vacuum. I also tested the water a couple weeks ago, and every thing (nitrates, nitrites, pH, etc.) checked out OK.

Answer
Hi Megan,
 It could be a disease called velvet, but it is pretty hard to say without actually seeing the fish.  Velvet looks like a very fine dusting on the fish.  Sadly, it is very possible that the disease came in on that pair of fish that you recently added to the tank.

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>