This means the system is not taking it all up, in this case you can dose the nano size vials.
If booster is left in the system the corals have had enough of a feed, bit like us if we order that 20 oz T bone but knew an 8oz sirloin was enough
If you also leave the skmmer running while doing the booster dosing this can play havoc as you coat the insides with neat Omega 3.
Hope that helps