...it is very image dependent. And the images you have on this thread look pretty tough except for the 2 that Mandrake posted. They would be a good place to start.
I'm not sure what Elements can do but here is how I would start.
Make a layer copy so you aren't working on the background layer.
make a new layer under the one you just made
fill the new layer with a bright color that is obviousley not part of the pic
go to your copy layer and add a layer mask.
Now your fore and background colors become balck and white. Where ever you paint black will become transparent - you'll see the bright color below. Where ever you paint white will be opaque - you will see what you want to keep.
Blow it up to 200% or better so you can see what you are doing and begin to paint transparency along the outside edge of what you want to keep (the fish), You can paint with whatever tool you like. Usually a paint brush, or making selections with the magnetic lasso and filling them with black as you go. I usually use some of each depending on the detail and contrast at the lines.
If you make a mistake - and you will - just paint white where you went over the line and you can repair it easily. When you have a tranparent ring around the object, it's easy to clear out the rest with selections and fill with black.
When you have what you want, remove the layer mask (apply) and you can select you object and have your way with it.
There are many more details for fine tuning but this is the down and dirty version and it works well with most images.
It can be a time theif but the more you do it the faster you'll get.
Be sure to show us your finished product!
Have fun!
