You basically need to cycle the tank first before adding any fish as I mentioned already that ammonia is toxic to fish and would recommend you to read the fishless article to get an understanding what needs to be done and why.
Cycling Your First Fresh Water Tank What is Cycling and Why is it Important? Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable...
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I understand this is a big learning curve and a lot of information to take on, especially when the chap at the fish store gave you advice about putting fish in after x number of days and letting the tank run for a few days or whatever jumbo jumbo he spoke of.
If you add a betta before cycling the tank, and don’t watch the water parameters then chances are your betta will suffer and possibly develop illnesses and worst case but likely scenario, die.
The bottle of safe start is basically bottled bacteria but it’s not enough just to pour this in and then add fish.