15 ml shouldn’t touch that size tank as far as showing up phosphates,
You need to introduce photo gradually and increase the dosing till you get to where you want to be, or....
You can dump it in till your tank starts to turn green with skimmer off
I dose feed my coral on Sunday. It includes about 500ml of phyto. Not the strong stuff like PhytoFeast. When I use PhytoFeast I only use about 100ml. Along with phyto I add about 100ml of OysterFeast and 2 table spoons Reef-Roids. Aprox 600g system. .09=P 4=N