Sealed bag: the fish respires co2
into the water. This drops the ph. The fish poos. But the low ph keeps the pollution in the form of harmless ammonium.
Open bag: co2
, trapped in the water and held there by the pressure, gasses off quickly and leaves the water. PH then rises fast, causing non-toxic ammonium to become toxic ammonia.
This assumes non-chemist me has got the rises and falls the right way around: but this is what happens. It was ammonium, which doesn't smell and is harmless, and mutates rapidly to stinky, highly toxic ammonia.
(From the thread)