Please read about nitrogen cycle any tank cycling. There are stickies at the top of the getting started forum.
Safestart is a product that claims to contain live bacteria to jump start your tank. Lets say for the sake of argument that it does. It still takes at least a week at a bare minimum for that bacteria to take up residence and multiply to a point that your fish will be safe from amonia and nitrite in the water.
Some people, myself included, do not believe safestart or any nitrifying bacteria product that does not need refrigeration is actually live active bacteria but probably does contain bacteria spores, in which case it will take several weeks or more to cycle your tank.
Any product that claims you can dump it in your tank and it is then safe to add fish is just plain false advertising.
Some people, myself included, will cycle a tank with fish but it takes a hardy fish to survive the conditions and in my case a good bacteria source to limit the amount of time it will take. Safestart is something I would not even consider using.