I believe the canonical response is "Move up to where they respect your raises".
How do
I deal with fish? I'm nice to them. I encourage them. I'm as friendly as possible. When they lose a big pot to me, I tell them they were just unlucky or I apologize for sucking out on them. I encourage them to reload and try again.
Fish making mistakes is how you make your money. Look up the "fundamental theorem of poker".
If you are losing money to fishy mistakes, one of two things is happening:
- You are on the wrong end of short-term variance, or
- You are playing worse than they are.