Gasping fish does not automatically mean that your plants are receiving good CO2 levels

If my fish are gasping, it must mean that my tank has high CO2 levels right?

If fish are gasping at the surface, it is a sign that there is too much CO2 in the aquarium at that time due to CO2 buildup. Immediate actions that can be taken are to aerate the tank and do a large water change, which will usually restore the CO2 levels immediately.

Despite the gasping fish, this does not mean that your plants are getting enough CO2 throughout the day. This is a huge paradox that is difficult for newer planted tank aquarists to accept. Let us examine it with the help of the following graph.

The blue line shows the CO2 buildup typical of tanks that have a combination of poor CO2 off-gassing (for people who are afraid of losing CO2, so they have no surface agitation or circulation) and low injection rates.

Say you start CO2 injection at 10am in the morning, and the aquarium lights come on at 11am. CO2 levels will drop as plants begin to take up the CO2. Since CO2 uptake by plants is greatest at the beginning of the light window, this initial dip is large. CO2 then builds up throughout the day. The aquarist may notice that the drop checker turns green or yellow by midday. (2 p.m.). As the day progresses, CO2 may build to lethal levels (3:00 p.m.), causing fish to gasp for air. Despite having a green Drop Checker at midday and gasping fish at the end of the day - the CO2 levels in this tank were actually sub-optimal for most of the day.

The ideal graph of CO2 levels in the tank should instead look like the following.

We want a quick buildup of CO2 levels at the beginning of the day, and yet have CO2 saturation drop off steeply at optimum levels. This can only happen if there is a combination of high injection rates and high off-gassing rates in a tank at the same time. A high injection rate without a high off-gassing rate - the tank would just quickly go to lethal CO2 levels. Good surface agitation and agitation will increase the rate of CO2 off-gassing to the atmosphere as CO2 levels rise. This combination allows us to have optimal CO2 levels early in the day, but never exceed levels that are harmful to the animals.

How to have better off gassing / gaseous exchange in a tank?

Have adequate surface agitation (filter outflow near surface), a clean water surface, and circulation between top and deeper layers of water. I use surface skimmers on my tanks to keep the water surface clean and draw oxygenated water into the filter. When there is adequate gaseous exchange, CO2 can be injected quite vigorously in combination with good O2 levels without fish gasping at the surface.