As Salaam Alaikum ,

I have fixed some amount in a bank. Iam wondering if i can use the interest earned. I want to use this amount after giving away 20% Khums , for charity work

If it is fixed in non Muslim bank and in non Muslim country you are allowed to get interest
Khums is applied on saving yes. Khums has two parts half goes to your marja or to his representative or the institution which has permission from your marja to collect the khums and half goes to poor and practicing Syed