It really depends on how sensitive is your fish. I have nano tanks where my drop checker would go yellow within a day, and the fish displayed no signs of distress. But those are tetras, loach, danios, betta... not the most finicky of the fish. My shrimp looked a little disappointed when I added too much CO2 too fast, but were fine the next day.
Either way, it's not the CO2 levels that are a problem (at least until you overdose it so much that your fauna starts to suffocate). What stresses out the fish is the rapid pH swing. pH changes happen in nature all the time over the course of a day (as plants consume carbon and the sunlight varies), so as long as you're not going from 8 to 6 in a couple of hours, your fish should be OK.
as far as algae, I can't really comment; IMO, there are too many factors that contribute to algae growth to reliably tie it to CO2 variance.