PnP paper would be wonderful. How fine do those finger nail polish detail pens come? They may be just perfect for this type of thing.
The fish tank bubbler just goes into the salt water to keep it churned up.
An electro forming kit would be wonderful for this but most don't have one. I have one but it is at a friends house.
Christa just told me that your copper plates should be close to the same size and works well if you have them close together in the solution. Mine where not on my first try and it worked ok, but maybe you will get a deeper etch quicker if they are so it's worth a try.
This is the salt water after just one etch YUCK but it isn't dead yet so I will use it again. You can pour it out into something else and let the yuck settle out then pour the good stuff back in.