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

Excessive CO2 levels are stressful towards livestock

Fish should be added to the tank when the CO2 is switched off. The sudden increase in CO2 levels can stress many species of fish.

When adjusting the CO2 in a new tank, always adjust the CO2 while you are near the tank and can observe the reaction of the animals, which will tell you if the levels are OK or too high.

Different fish species will also have different tolerances to CO2 levels. Fish used to high oxygen, cold, fast flowing streams will be less tolerant of CO2 than fish from acidic peat bogs. In general, larger fish are more sensitive to CO2 than smaller ones, and discus are particularly sensitive, especially given the higher water temperatures at which they are usually kept. Discus gets sensitive when CO2 levels approach 40ppm. Cardinal tetras are very resistant to CO2 levels and may show no signs of discomfort even at 60+ppm of CO2.

A useful way of monitoring fish is to compare their behaviour when CO2 is not switched on with their behaviour when the tank water is saturated with CO2.

Signs that CO2 saturation is too high:

Lowered Activity: Fish that are normally active may hide or become less active, sluggish, show signs of labored breathing, delayed reaction time to feeding.

Position Change: Fish will change their natural positions to favour positions closer to the top of the aquarium or towards areas of high current. Even snails and shrimps will hang out near the top of the tank even 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 the surface are very serious signs and a water change or rapid aeration should be carried out immediately to reduce 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.