Yeah O2 might not make sense.
The thing is - other than a gas, little else makes sense for a 5 minute reaction.
But thinking more, those symptoms really match carbon dioxide and carbon monoxide poisoning.
Could be high tank CO2 level + CO / CO2 released from decaying algae as well when disturbed.
Easy enough to test for ammonia. A spike that can kill in 5 minutes would certainly show up on a basic test kit.
Test PH levels before/after, and in an aerated glass. If that changes between samples, you may have an O2/CO2 issue.