Idiom to describe something that has little to no effect?
We will need a little more context to clearly understand what you have in mind. For now, a drop in the ocean or a drop in the bucket comes to mind.
a drop in the ocean (British, American & Australian) also a drop in the
bucket (American)
a very small amount in comparison to the amount that is needed
A hundred thousand may seem a lot but it's a drop in the ocean compared to
the millions that need to be spent.
This expression comes to mind:
like extinguishing a forest fire with an eyedropper