Um...the same as a fish in a tank without a DIY CO2 system. Or actually
perhaps a bit longer, since the only reason for injecting CO2 is
because the tank has a large number of plants which invariably improve
water quality.

DIY CO2 doesn't hurt fish as long as you don't do anything stupid. I
was careless - I should have waited until morning to mix a new batch so
I could monitor the change throughout the day.

-Daniel