Hi Bea,
You've either been really lucky or know some secret that we dont. when you have a new tank there is no "Good" bacteria devloped, so you have to ease into it until this has established and this will then be able to maintain a normal stocked fish tank. if you just chuck in a load of fish the toxins will just build up until they're all wiped out. did you use things from an already established aquarium like for example water, filter media, gravel, etc.