I think the odds are very low that a 75 gallon tank could get that much pH drop from a single source of CO2 entering from an air stone. But, I haven't tried it, and you did, so maybe it isn't improbable at all.
Seems unlikely. I have a 24" tall 5" diameter CO2 reactor run by a mag pump at a bubble count that is closer to 10 bps and my pH drops from 7.4 to 6.6 but it takes some time to get there. An air stone is pretty inefficient when it comes to injecting CO2 into a tank.
Hoppy, I hooked up a small powerhead and aimed my bar up a little and I'm at 6.4 now... I read a sticky that made think. I didn't have ANY surface disturbance at all. I think that was the big problem. Lost an oto I should have known better then to add a fish so soon..... I waited 36 hours to add a fish and when i tested the water, it was perfect! 36 hours is to soon i know but I have added fish after 24 hours and never had problems. I learned my lesson
MarkMc, I do have the stone at the bottom of the tank and, under the output bar. I am thinking that there wasn't enough surface distrubance to release the co2. I made a few changes and the PH did go up. Very odd... If that is the case, maybe this 10lbs tank will last a full year! jk. Wishfull thinking,.. Waiting fot my drop checker!!