Is your brine shrimp freeze dried or frozen? 

What I do with freeze dried food, is scoop a little bit of tank water into a plastic tub that is ONLY used for aquarium food (You don't want to use something that you intend to store or eat human food out of later).

Then break enough off for the fish and let it soak in the tub for a few minutes, then simply pour it back into the tank

I keep my plastic tub on top of the tank so it doesn't get confused with or used for anything else