So sorry that happened to you. The best advice is to always be at home near your tank when you first set-up and adjust your CO2. Drop checkers take hours to register all the CO2 in your water. Your fish react immediately. There are so many things that you need to be able to watch - faulty needle valve, back pressure of the diffuser, etc.
One safety measure is pH controller. They measure the pH quickly. The change of pH of your tank is a direct indicator of how much CO2 is in your water. I have the Milwaukee SMS122 pH controller. The CO2 regulator plugs in to the pH controller and I set the controller not to go below a certain pH. If varies from tank to tank because of the dissolved solids in your water. Since it will turn off my regulator (CO2 output) when the pH gets to the setting I dial in, my fish cannot get too much CO2. When I first set up a tank I watch all of this for hours.
You also have to be careful when you add new fish to your tank. Any new additions could be sensitive to all the pH in your water. You could notice them at the top gasping for air. If you see them having trouble you can do an immediate water change, turn your CO2 down temporarily, and add an airstone. New fish will get used to the CO2 but it can take them a few hours.
I've had CO2 for many years now. One of my pelcos is 12 years old. Once you get your CO2 balanced your fishes can live a long and happy life in an aquarium that rivals planted lakes and rivers. Remember - the CO2 helps the plants infuse so much oxygen into the water that many times we can see it pearling!
What fish wouldn't like that environment?!