If i'm correct you should add 0.5 mL to your 1 gallon tank, and I would say about 1.25 mL to your 2.5 gallon. It will not be extreme if you end up putting a little more in than you are supposed to. I'm not 100% because i haven't slept so my brain is being slow.
It is prime, it can't be divided by anything, but itself and one.
let
x--------> the monthly fee
y--------> the administration fee
we know that
--------> equation 
--------> equation 
Multiply by
equation 
------> equation 
Adds equation
and equation 

Find the value of y

therefore
the answer is
the administration fee is equal to 
When we factor out something, we divide the factor by the equation. For example if we were to factor 3 out of 6, It will be 3(2). We got this answer by dividing 3 and 6. Using this concept:
![\frac{-1}{4}[( \frac{-1}{2} / \frac{-1}{4}) - ( \frac{5}{4} y / \frac{-1}{4})]](https://tex.z-dn.net/?f=%20%5Cfrac%7B-1%7D%7B4%7D%5B%28%20%5Cfrac%7B-1%7D%7B2%7D%20%2F%20%5Cfrac%7B-1%7D%7B4%7D%29%20-%20%28%20%5Cfrac%7B5%7D%7B4%7D%20y%20%2F%20%20%5Cfrac%7B-1%7D%7B4%7D%29%5D%20%20)
Simplifying:

Hope this helps!