Red gills? or normal?

i feel really bad about this but here it goes. i just got 3 guppies in what i thought was a cycled tank. 2 just died and the third looks healthy except for one thing- its head and gill area are starting to turn red, more of a mild pink. this guy was consistantly hiding behind the filter until the other 2 died, now he's been much more active. its been about a week. im nervous because the other two fish had symptoms of ammonia poisoning, but i just got a meter to read levels and theyre at 0, catastrophe avoided. (i add nitrifying bacteria every day, cant hurt) nitrites were at 0 too. then my water was very basic, so i had to get some easybalance to level out the ph, over two days its at 7.8. if you live in the northeast, youll know that its been hot as hell out, and the temp has been fluctuating in the tank between 76-82, i just got this under control using a heater at night and a fan blowing over the top during the day. i really just want this to take its course naturally but i feel guilty for these fish, i dont want this last one to die. does anyone know if this sounds like ammonia, nitrite, disease, stress? the last fish that died had a few red spots on it that looked like blood, is that ammonia as well?