float for 15 then add fish? nope sorry. you need to do more than that.
one way is:
float for 15-20min to get water in bag same temp as tank, then slowly add tank water to bag, (we add about a a shot or 2 every 10-15min) till the bag is full, dump half water down the drain (not in your tank) then repeat adding water till bag is full again. then add fish to tank. with the tank lights off. do not let the water from the bag get into your tank.
you can also use the
drip method to add fish.
since it seems you're new to this hobby, you may want to read a good book or spend some time on the board learning this hobby.
try to get an id on the fish that you don't know what they are. don't add too many, if you don't know the filtering info, the tank may not be able to add a higher bioload then what it has.
before you buy a fish, research the fish, know if it will get along with what you have, know what you need to do to care for it, ect.
welcome and good luck!