The death of the plants could have caused an 02 crash. One of the experts is sure to give you a better answer. When I applied chemicals to my pond to treat Elodea I was advised not to treat more than 20-25% of the pond at one time to avoid an 02 crash.

JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)