I once turned off my pump to do some work, and did not turn off the solenoid on my CO2, to shut of the CO2 going into my reactor. After a few hours of cleaning, and not realizing the my CO2 reactor had aparently about filled up with CO2, I turned the pump back on. pH dropped from 6.9 to 6.6 in less than five minutes, and was still falling when I started an emergency 50% water change and massive surface agitation to release the CO2.
I'm still grateful I didn't lose every fish in the tank.
Steve - 33g reef and a 180g planted in need of a re-scape.