So i need to add 2 teaspoons of conditioner for every 10 gallons of water in a tank. i have a 2.5-gallon tank, how much conditioner do i need to put in the tank?i don't want my fish to die.

So, you need 2 teaspoons of conditioner for every 10 gallons of water. 

This is really just a simple equation:

2/10=x/2.5

Now, there are several ways you can really solve this, but i'll give you the basic version. You know for 10 gallons, you need 2 teaspoons. Divide, this means for every 1 teaspoon, you need 5 gallons. This here is a simplification. 

Now, in order to get from 5 gallons per 1 teaspoon to 2.5 gallons per X teaspoons, you have to figure out the differences in what you know. 2.5 is half of 5, and what you do to one side of an equation, you do to the other. The first half was divided by half, so divide the second half (1 teaspoon) by half as well. 

1/2=.5, AKA half.

2 teaspoons of conditioner for every 10 gallons
For 2.5 gallons, you need .5 teaspoons, or 2.46 ml.

Your answer is .5 teaspoons, basically HALF of a teaspoon.

~Hope this helps!