For future reference... if the medicine is 1 teaspoon for 10 gallons... then I would put ten teaspoons full of water into a cup, add the 1 teaspoon of medicine, mix and then put the required number of teaspoon fulls into a smaller tank - ex: 2 1/2 teaspoons full for a 2.5 gallon tank.
It's easier to use a syringe to measure the medicine and the water with, then use the syringe to administer the mix. The syringe has ml measurements marked on it.