With that many fish etc. dieing in that short of period of time I highly doubt that it is a predator hitchiker. Particularly since you say they had no other marks on them other than puffy gills.
A couple of possibilities come to my pea brain. Did you by any chance have a power outage or loose circulation for a period of time? Do you run a skimmer and perhaps shut it off overnight. The reason I ask is because when I had a breaker trip and lost circulation overnight in one of my tanks I lost 4 fish (out of 7) and it was from lack of oxygen in the water. For the most part the fish I lost were large however on very large one (my Naso Tang)survived. Some fish are way more suseptable to an oxygen drop.
The next thing to explore is water quality. I know you tested yours but could you please let us know what brand of test kit you are using and what your #'s for Ammonia, Nitrate, Nitrite, PH, calcium, alk/dkh, and magnesium are. Sometimes it can be an inbalance in chemestry however I do not suspect this in your case.
Of course the last thing is some kind of disease that only effected some of your livestock and that list is way too long and complicated to try and guess at.
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