i'm still guessing, but if it doesn't stay dissolved in the bottle and similar products are available in powder form, it may not stay in the water column.
clean the sand bed. Don't dose anything unless you are sure you need to do so. Keep it simple, it's a new tank.
poly lab one is two birds with one stone. It's replaces Alk, CA & Mg that the corals are using up, plus its a form of carbon dosing - read up on that - which is adding organic (not activated) carbon into the tank that feeds bacteria that uses up nitrogen, and then that bacteria gets removed from the tank by the protein skimmer. The net effect it lowers NO3. Both these are techniques for mature tanks and require some advanced testing skills/kits. I wouldn't recommend either for a new tank given that water changes by themself should be all you need.
keep updating the progress.