Relax!

I am not sure what you are going to sue her for. do you feel you were discriminated against, or/and your human rights were violated? My guess is that cashier is a true animal lover and she was going overboard to protect "fish" rights. A little comical, but definitely not to the point where she deserves to be dragged into a lawsuit. If you don't like her, don't go to that store again. Somehow I feel that woman can actually be very helpful in giving you real advice when you need it.