nutrient question

my plants are 19 days old and i was wondering if i could start adding nutes at a low dose and if so do i feed them every time i water?

yes you can..although most composts(if that is your medium)contain enough nutes for around 1 month of good growth..
Have the round cotyledon leaves dried up yet?
If so, SLOWLY build the nutes up from 1/6th suggested dose to around 3/4 of the suggested dose.