I use Flourish twice a week in my two guppy tanks, with adults, juveniles, and fry. Primal is still actively producing a healthy amount of fry every month, and I have noticed no deaths or deformities.
It depends which of the fertiliser series you are using. Flourish Comprehensive Supplement for the Planted Aquarium is safe. But I would not use Flourish Excel in any tank with fish, fry or adults.