Looking for Helium Isotopes, consistently 10 million units every week.
Fuel needs to be at Jita. 95% JBV
10M units at 816 ISK/unit for Helium Isotopes as of right now is 8.167B ISK. With a 5% decrease in price value (what you are asking for) is a decrease of 408M of the total value for Helium Isotopes. That is also a lot of helium isotopes for a week.
If you are referring to fuel blocks (Helium Fuel Blocks not Isotopes) the numbers are worse:
10M x 20300 ISK/fuel block = 203B a week minus the 5% price decrease (what you are asking) thats a negative 10B right there.
You are better off creating buy orders and hope for the best.