I'm with Hypancistrus, I'd rather have my ferts dissolved even BEFORE I introduce them to the aquarium. That said, when I dose macros I measure my 1/4-1/2 teaspoon of the powder, put it in a cup, and add HOT water. This dissolves it very fast, and the small amount of hot water doesn't affect the tank temperature. I do the same with micros, but it's 1/8 teaspoon. the only one I premix is phosphates, and my solution is strong enough that 4 mL raises my 29g by 1 ppm.