Koi Angelfish not looking so good. any suggestions?

[Plecostomire]

I have a new 29gal tank and i stocked it with 2 koi angelfish and 6 guppies. i know you arent suppsed to have them together but the angelfish are still too small to eat the guppies and the guppies will be transfered to another tank with more similar sized fish with a few weeks.
Back on topic. The angelfish were doing fine until last night, but when i woke up this morning one was upside down on the bottom of the tank, still breathing. it is putting no effort what so ever into swimming, staying upside right, or anything but breathing. it simply lets the current and gravity take over.
All my other fish in my new tank are okay still. i tested the ammonia and pH levels, the pH was about 7.8 and the ammonia was at very close to zero, if not zero. I dont know whats wrong with her, anyone have any ideas?

 

[OrangeCones]

I'd test for nitrite and nitrate also, not just ammonia, and simply due to a reading of ANY ammonia, I'd do a 50% water change and continue to monitor. Based on what you describe, I think the fish is too far gone

How long has the tank been set up?

 

[Plecostomire]

PLEASE HELP! i would really like to avoid having this fish die

 

[Plecostomire]

haha, i didnt see your post. the tank has been set up for a few days. i had it set up and running, having used "Start right" stuff to get the biological filter started, for about 30 hours before putting the fish in. it has been about 3 or 4 days since i got them, and they showed no sings of stress or anything like that until this morning.