Some things just don't make sense;
a. If half the food was gone, they would have been some excess food lying around in the tank as i couldn't imagine even all my fish eating a half of the smallest tubs of food available in my lfs in a couple of days, and i have a fair amount of fish.
b. On top of the excess food, the dead bodys would have created ammonia in the tank as things like this does not take long at all to start rotting and causing ammonia.
c. If the hex tank had no fish in it before, there is no way it would have been cycled when you put them in. Adding such a large number of fish all at once even to a mature tank would have caused some water quality issues.
d. Just because there is no ammonia and nitrites present does not mean the tank isn't cycling, you need to have nitrate stats to rule out wether or not you had a healthy bacterial cycle going.
e. a PH of 0.2 is VERY low, somthing certainly does not seem right here, what is the PH stat in your big tank?
The stats just don't seem to be adding up to me, i do not believe you are lying at all as you have no reason to do so but either way, the stats just don't seem right. Test your water quality for ammonia, nitrites, nitrites and PH with a brand new accurate test kit, make sure it is a new test kit as ones that have been exposed to moisture, left out in the open in hot temps, or simply ones that are going old can all give misleading wrong stats, so i suggest you get a new kit for all these stats just to make sure whats going on in your water quality.
What are the tank dimensions for your 150gal tank?