So I have been scuba diving for nearly 10 years at this point and have been nitrox certified for nearly as long. For those that do not know Nitrox is a mix of gasses for scuba diving where some of the nitrogen has been replaced with oxygen. This is done because nitrogen tends to be the gas that causes symptoms of decompression sickness so by adding more oxygen you reduce the chance of getting those symptoms and allow yourself the ability to stay down longer before needing to worry.
While it sounds like this is nothing but benefits the increase oxygen puts you are risk of oxygen sickness which can cause convulsions if you go to deep. To remedy this you are required to check your oxygen percentage after having your tanks filled and before you dive. To do this you use a nitrox analyze which connects to your tank and measures the oxygen coming out. They don't need to be too accurate as you round up to the next highest percentage of oxygen however still cost nearly $200-300