Now to try and help, did you see the initial spikes? Unfortunately, I didn't start recording the test results until 7/25. I see you said they were reading zero, just wondering if you ever saw them not be zero. Yes, I tested daily until they were zero. That seems like a very short cycle, did you add live rock? I agree, very short cycle but I only used about 5 lbs of LR the other 70 or so is marco rock, which is why I used the chromis. No nutrients, no cycle. Are your test kits expired? Nitrate was but was replaced. I had a test kit consistently show zero even though I could visually see some of my corals looking off. I ordered a different brand and found it was off. Couldn't hurt. Do you recommend a brand?
What kind of QT process are you using? It sounds like you are adding directly to the tank. This can work, however it puts your entire tank at risk and doesn't let you quickly treat for disease if you do find something. Something that quick that only impacted the three fish would leave me to believe a swing in ph or salinity. I wouldn't think disease at least initially because you would most likely see some signs.
I have a QT tank waiting to be set-up. I still need a few things.