You have no need to switch to Nitrox. Nitrox complicates things if you don't know what you are doing when you start making deeper dives. Nitrox can actually become poisonous to your body at a shallower depth than standard air because of partial pressure, and it all varies with how good of shape you are in etc. Your body has to metabolize Oxygen, unlike Nitrogen and Helium. There is a place for mixed gas in diving, and it isn't at 25 ft. For example we use Nitrox on working dives around 50 feet when the diver is working hard as a safety factor so he doesn't overbreathe his hat. But we never exceed the U.S. Navy No Decompression schedule unless of course a decompression chamber is on-site and we can safely do that. I am familiar with the dive computer that you are using and chances are, it is probably right, however recreational dive tables are a good guide and stay on the safe side. My advice to you would be, stick with air, plan your dive in accordance with the dive tables you were instructed with, and stick to the plan. You can wear your dive computer and use it for fun as supplemental information, but don't trust it with your life.
