Sorry to hear about the loss of your Guppy
. I just lost three Guppy
A few questions to start out: what are your water parameters, exactly? How old is your tank? How many fish do you have total?
Ideally, you should have 0ppm of both ammonia and nitrite, and very low nitrate. I personally do a water change every time my nitrates hit 15ppm, but others let it get a little higher.
It sounds as if your tank is new since you mentioned foggy water. One of the biggest culprits of foggy water is a bacterial bloom, which usually occurs shortly after starting a tank. If this is the case, you should try to move all of your fish to an established tank, since most of your fish will not survive the cycling process.
I can count at least 10 fish in your 10-gallon tank: 4 guppies (including the one that died), 4 neons, and 2 cories. I think you have too many fish for such a small tank. At most you should have about 6 small fish, though since the neons a really small, you could probably do 5-6 neons with the cories. Is there any way you can get a second 10-gallon and have two tanks running? Make one a Guppy
tank, and one a neon tank, and put one cory at the bottom of each. Just a thought. :)
About the "sumbissive" Guppy
: the symptom of making his tail narrow is called clamped fins, and it isn't a good sign. It does sound like he's being picked on, so moving him if possible would be great. Keep a close eye on him and see if he will start eating once he is safe from your aggressive Guppy
About the fat one: could it be a pregnant female? If so, congrats!! If it's a male, is he eating more than his fair share? Is there a lump or growth rather than just being universally fat?
Keep up the good work of observing your fish. Sounds like you are staying in tune with your tank, which is great.