Need help plz look and help me out here

Hi im abit confused or stoned to much to even think straight,
i want use the product cha ching
the thing is im confused on how much i should use in my 50 liter res
it say: 1/4 to 1/2 tsp per gallon
am i right in thinking 50 liters is 11 gallon
there for i should add 5 full tsp +1/2 tsp
am i correct in that or am i totally wrong

calculator
http://www.metric-conversions.org/volume/liters-to-us-liquid-gallons.htm

It's 2 1/2 teaspoons per 10 gal. Using 1/4 teaspoon per gallon.

thank you beuffer

i have a 50liter res and i rubbish with workin in gallons lol im in uk not us

also when should i start using CHA CHING what week ??