The fix a flat can fix a slow leak in the tire as long as the leak is from a small hole in the tread area of the tire and the tire is not leaking from the bead, the sidewall or through the valve stem.
Before adding fix a flat to your tire take a spray bottle and add 3 to 4 drops of some liquid dish soap to the spray bottle and then add some water to the soap in the spray bottle and shake it up to mix it.
Then spray the soapy water solution around the bead "rim" of the tire. The bead of the tire is where the rim attaches to the tire.
You need to spray both sides of the rim where the tires bead is and also spray the soapy water onto the valve stem and around the whole tire.
You need to jack the car up so that the tire is off the ground as well so you can spin it around as you spray the tire with the soapy water to find the slow leak.
If the leak is just a slow leak in the tread area it would be much better to just take the car or tire to a tire shop and have them either plug the tire or patch the tire since fix a flat can cause a real mess when you need a tire changed.
If the leak is in the valve stem a tire shop can change the valve stem for cheap as well or they can also clean the rim and reset the bead of the tire to make the leak stopped.
Fix a flat should only be used in an emergency situation where there's no tire shop available.