How do I know if my CO2 levels are too much for my fish?

Excessive CO2 levels are stressful towards livestock

When adjusting CO2 in a new tank, CO2 should always be tuned while one is near the tank and able to watch livestock reaction, which can reveal if the levels are OK or too high. This assumes that you have had the opportunity to observe the livestock at length, over days / weeks before tuning CO2, and that the livestock have acclimatised to the tank (i.e not newly introduced).

Different species of fish will also have different tolerances to CO2 levels. Fish used to highly oxygenated cold, faster flowing streams will be less CO2 tolerant compared to fish from acid peat bogs. Generally, larger fishes are more sensitive to CO2 than smaller ones and Discus are especially sensitive, especially given the higher water temperatures that they are usually kept in as well. Snails and shrimp are sensitive to excessive CO2 levels as well.

One useful way to check on livestock is to compare their behaviour when CO2 is not turned on VS when the tank's water is CO2 saturated. 

Signs that CO2 saturation is too high:

Lowered Activity: Fish that are usually active hide or are less active, sluggish, signs of labored breathing, delayed reaction time in feeding.

Position Change: Fish change their natural positions to favour positions closer to top of tank level or towards high flow areas. Even snails and shrimp will hang out near top of the tank though it is not a natural position for them.

Waterline breach: Snails/shrimps try to breach the water-line.

Decreased brood rates: Applies to CRS and sensitive shrimp

Erratic behavior: Fish showing erratic behaviour and/or gasping at surface are very severe signs; water change or rapid aeration should be done to immediately slower CO2 levels.

​Different species react differently, so it pays to observe and learn what constitutes "normal" behavior for the particular livestock you are keeping. Good gaseous exchange is best counter to excessive CO2 levels. Read more on tank gaseous exchange here.