I am a new aquarium owner and have been running what I think is a 40 gallon tank for about 2 months now. I started with a couple of small (1-2 inch) goldfish from my folks' outdoor pond (which are the offspring of some feeder fish I threw in there a couple of years ago to eat mosquito larvae). They are very hardy, if not really gold. I then added 5 neon tetras, and 2 polka dot mollies who are dong well. Then I added 3 guppies, 2 male and 1 female about 3 weeks ago, the 1st male died after about a week (I had suspicions that the female might have been responsible as she is aggressive with them). 2 days ago I found the 2nd male dead, his tail had been chewed by the female but otherwise there was no outward sign of disease or distress. Now the female seems fine but I've noticed some rust coloured spots around her head and am concerned as I don't believe they were there initially. It might be natural colouration, I've been feeding them tropicla flake that is supposed to enhance colour, I just don't know. I've read all of the articles on this site that I could find relating to fish illness and no one has mentioned any similar symptoms that I've found. If anyone has an idea I'd welcome it!